diff --git a/composer.json b/composer.json
index ec7ad4e1..e2318963 100644
--- a/composer.json
+++ b/composer.json
@@ -41,6 +41,7 @@
         "bundled": true,
         "commands": [
             "config",
+            "config add",
             "config edit",
             "config delete",
             "config create",
@@ -50,7 +51,8 @@
             "config list",
             "config path",
             "config set",
-            "config shuffle-salts"
+            "config shuffle-salts",
+            "config update"
         ]
     },
     "autoload": {
diff --git a/features/config-add.feature b/features/config-add.feature
new file mode 100644
index 00000000..90acbddf
--- /dev/null
+++ b/features/config-add.feature
@@ -0,0 +1,138 @@
+Feature: Add a constant or variable to wp-config.php file
+
+  Scenario: Add a new constant to wp-config.php
+    Given a WP install
+
+    When I run `wp config add NEW_CONSTANT constant_value`
+    Then STDOUT should be:
+      """
+      Success: Added the constant 'NEW_CONSTANT' to the 'wp-config.php' file with the value 'constant_value'.
+      """
+
+    When I run `wp config get NEW_CONSTANT`
+    Then STDOUT should be:
+      """
+      constant_value
+      """
+
+  Scenario: Add a new variable to wp-config.php
+    Given a WP install
+
+    When I run `wp config add new_variable variable_value --type=variable`
+    Then STDOUT should be:
+      """
+      Success: Added the variable 'new_variable' to the 'wp-config.php' file with the value 'variable_value'.
+      """
+
+    When I run `wp config get new_variable`
+    Then STDOUT should be:
+      """
+      variable_value
+      """
+
+  Scenario: Add a raw constant to wp-config.php
+    Given a WP install
+
+    When I run `wp config add WP_CUSTOM_CONSTANT true --raw`
+    Then STDOUT should be:
+      """
+      Success: Added the constant 'WP_CUSTOM_CONSTANT' to the 'wp-config.php' file with the raw value 'true'.
+      """
+
+    When I run `wp config list WP_CUSTOM_CONSTANT --strict --format=json`
+    Then STDOUT should contain:
+      """
+      {"name":"WP_CUSTOM_CONSTANT","value":true,"type":"constant"}
+      """
+
+  Scenario: Fail when trying to add an existing constant
+    Given a WP install
+
+    When I run `wp config add TEST_CONSTANT test_value`
+    Then STDOUT should be:
+      """
+      Success: Added the constant 'TEST_CONSTANT' to the 'wp-config.php' file with the value 'test_value'.
+      """
+
+    When I try `wp config add TEST_CONSTANT another_value`
+    Then STDERR should be:
+      """
+      Error: The constant 'TEST_CONSTANT' already exists in the 'wp-config.php' file.
+      """
+
+  Scenario: Fail when trying to add an existing variable
+    Given a WP install
+
+    When I run `wp config add test_variable test_value --type=variable`
+    Then STDOUT should be:
+      """
+      Success: Added the variable 'test_variable' to the 'wp-config.php' file with the value 'test_value'.
+      """
+
+    When I try `wp config add test_variable another_value --type=variable`
+    Then STDERR should be:
+      """
+      Error: The variable 'test_variable' already exists in the 'wp-config.php' file.
+      """
+
+  @custom-config-file
+  Scenario: Add a new constant to wp-custom-config.php
+    Given an empty directory
+    And WP files
+
+    When I run `wp config create {CORE_CONFIG_SETTINGS} --skip-check --config-file='wp-custom-config.php'`
+    Then STDOUT should contain:
+      """
+      Generated 'wp-custom-config.php' file.
+      """
+
+    When I run `wp config add NEW_CONSTANT constant_value --config-file='wp-custom-config.php'`
+    Then STDOUT should be:
+      """
+      Success: Added the constant 'NEW_CONSTANT' to the 'wp-custom-config.php' file with the value 'constant_value'.
+      """
+
+    When I run `wp config get NEW_CONSTANT --config-file='wp-custom-config.php'`
+    Then STDOUT should be:
+      """
+      constant_value
+      """
+
+  Scenario: Additions can be properly placed in wp-config.php
+    Given a WP install
+    And a wp-config.php file:
+      """
+      define( 'CONST_A', 'val-a' );
+      /** ANCHOR */
+      define( 'CONST_B', 'val-b' );
+      require_once( ABSPATH . 'wp-settings.php' );
+      """
+
+    When I run `wp config add SOME_NAME some_value --anchor="/** ANCHOR */" --placement=before --separator="\n"`
+    Then STDOUT should be:
+      """
+      Success: Added the constant 'SOME_NAME' to the 'wp-config.php' file with the value 'some_value'.
+      """
+    And the wp-config.php file should be:
+      """
+      define( 'CONST_A', 'val-a' );
+      define( 'SOME_NAME', 'some_value' );
+      /** ANCHOR */
+      define( 'CONST_B', 'val-b' );
+      require_once( ABSPATH . 'wp-settings.php' );
+      """
+
+    When I run `wp config add ANOTHER_NAME another_value --anchor="/** ANCHOR */" --placement=after --separator="\n"`
+    Then STDOUT should be:
+      """
+      Success: Added the constant 'ANOTHER_NAME' to the 'wp-config.php' file with the value 'another_value'.
+      """
+    And the wp-config.php file should be:
+      """
+      define( 'CONST_A', 'val-a' );
+      define( 'SOME_NAME', 'some_value' );
+      /** ANCHOR */
+      define( 'ANOTHER_NAME', 'another_value' );
+      define( 'CONST_B', 'val-b' );
+      require_once( ABSPATH . 'wp-settings.php' );
+      """
diff --git a/features/config-update.feature b/features/config-update.feature
new file mode 100644
index 00000000..f666f40e
--- /dev/null
+++ b/features/config-update.feature
@@ -0,0 +1,188 @@
+Feature: Update or add a constant or variable in wp-config.php file
+
+  Scenario: Update an existing constant in wp-config.php
+    Given a WP install
+
+    When I run `wp config update DB_HOST db.example.com`
+    Then STDOUT should be:
+      """
+      Success: Updated the constant 'DB_HOST' in the 'wp-config.php' file with the value 'db.example.com'.
+      """
+
+    When I run `wp config get DB_HOST`
+    Then STDOUT should be:
+      """
+      db.example.com
+      """
+
+  Scenario: Add a new constant when it doesn't exist
+    Given a WP install
+
+    When I run `wp config update NEW_CONSTANT constant_value`
+    Then STDOUT should be:
+      """
+      Success: Added the constant 'NEW_CONSTANT' to the 'wp-config.php' file with the value 'constant_value'.
+      """
+
+    When I run `wp config get NEW_CONSTANT`
+    Then STDOUT should be:
+      """
+      constant_value
+      """
+
+  Scenario: Update an existing constant then add it again
+    Given a WP install
+
+    When I run `wp config update TEST_CONSTANT first_value`
+    Then STDOUT should be:
+      """
+      Success: Added the constant 'TEST_CONSTANT' to the 'wp-config.php' file with the value 'first_value'.
+      """
+
+    When I run `wp config update TEST_CONSTANT second_value`
+    Then STDOUT should be:
+      """
+      Success: Updated the constant 'TEST_CONSTANT' in the 'wp-config.php' file with the value 'second_value'.
+      """
+
+    When I run `wp config get TEST_CONSTANT`
+    Then STDOUT should be:
+      """
+      second_value
+      """
+
+  Scenario: Update a variable with --type=variable
+    Given a WP install
+
+    When I run `wp config update new_variable variable_value --type=variable`
+    Then STDOUT should be:
+      """
+      Success: Added the variable 'new_variable' to the 'wp-config.php' file with the value 'variable_value'.
+      """
+
+    When I run `wp config update new_variable updated_value --type=variable`
+    Then STDOUT should be:
+      """
+      Success: Updated the variable 'new_variable' in the 'wp-config.php' file with the value 'updated_value'.
+      """
+
+    When I run `wp config get new_variable`
+    Then STDOUT should be:
+      """
+      updated_value
+      """
+
+  Scenario: Update raw values in wp-config.php
+    Given a WP install
+
+    When I run `wp config update WP_DEBUG true --raw`
+    Then STDOUT should be:
+      """
+      Success: Updated the constant 'WP_DEBUG' in the 'wp-config.php' file with the raw value 'true'.
+      """
+
+    When I run `wp config list WP_DEBUG --strict --format=json`
+    Then STDOUT should contain:
+      """
+      {"name":"WP_DEBUG","value":true,"type":"constant"}
+      """
+
+    When I run `wp config update WP_DEBUG false --raw`
+    Then STDOUT should be:
+      """
+      Success: Updated the constant 'WP_DEBUG' in the 'wp-config.php' file with the raw value 'false'.
+      """
+
+    When I run `wp config list WP_DEBUG --strict --format=json`
+    Then STDOUT should contain:
+      """
+      {"name":"WP_DEBUG","value":false,"type":"constant"}
+      """
+
+  @custom-config-file
+  Scenario: Update a constant in wp-custom-config.php
+    Given an empty directory
+    And WP files
+
+    When I run `wp config create {CORE_CONFIG_SETTINGS} --skip-check --config-file='wp-custom-config.php'`
+    Then STDOUT should contain:
+      """
+      Generated 'wp-custom-config.php' file.
+      """
+
+    When I run `wp config update DB_HOST db.example.com --config-file='wp-custom-config.php'`
+    Then STDOUT should be:
+      """
+      Success: Updated the constant 'DB_HOST' in the 'wp-custom-config.php' file with the value 'db.example.com'.
+      """
+
+    When I run `wp config get DB_HOST --config-file='wp-custom-config.php'`
+    Then STDOUT should be:
+      """
+      db.example.com
+      """
+
+  Scenario: Ambiguous update requests throw errors
+    Given a WP install
+
+    When I run `wp config update SOME_NAME some_value --type=constant`
+    Then STDOUT should be:
+      """
+      Success: Added the constant 'SOME_NAME' to the 'wp-config.php' file with the value 'some_value'.
+      """
+
+    When I run `wp config update SOME_NAME some_value --type=variable`
+    Then STDOUT should be:
+      """
+      Success: Added the variable 'SOME_NAME' to the 'wp-config.php' file with the value 'some_value'.
+      """
+
+    When I run `wp config list --fields=name,type SOME_NAME --strict`
+    Then STDOUT should be a table containing rows:
+      | name      | type     |
+      | SOME_NAME | constant |
+      | SOME_NAME | variable |
+
+    When I try `wp config update SOME_NAME some_value`
+    Then STDERR should be:
+      """
+      Error: Found both a constant and a variable 'SOME_NAME' in the 'wp-config.php' file. Use --type=<type> to disambiguate.
+      """
+
+  Scenario: Update with placement options for new constants
+    Given a WP install
+    And a wp-config.php file:
+      """
+      define( 'CONST_A', 'val-a' );
+      /** ANCHOR */
+      define( 'CONST_B', 'val-b' );
+      require_once( ABSPATH . 'wp-settings.php' );
+      """
+
+    When I run `wp config update SOME_NAME some_value --anchor="/** ANCHOR */" --placement=before --separator="\n"`
+    Then STDOUT should be:
+      """
+      Success: Added the constant 'SOME_NAME' to the 'wp-config.php' file with the value 'some_value'.
+      """
+    And the wp-config.php file should be:
+      """
+      define( 'CONST_A', 'val-a' );
+      define( 'SOME_NAME', 'some_value' );
+      /** ANCHOR */
+      define( 'CONST_B', 'val-b' );
+      require_once( ABSPATH . 'wp-settings.php' );
+      """
+
+    When I run `wp config update SOME_NAME updated_value --anchor="/** ANCHOR */" --placement=before --separator="\n"`
+    Then STDOUT should be:
+      """
+      Success: Updated the constant 'SOME_NAME' in the 'wp-config.php' file with the value 'updated_value'.
+      """
+    And the wp-config.php file should be:
+      """
+      define( 'CONST_A', 'val-a' );
+      define( 'SOME_NAME', 'updated_value' );
+      /** ANCHOR */
+      define( 'CONST_B', 'val-b' );
+      require_once( ABSPATH . 'wp-settings.php' );
+      """
diff --git a/src/Config_Command.php b/src/Config_Command.php
index 9fd9d0b7..5b01f6a1 100644
--- a/src/Config_Command.php
+++ b/src/Config_Command.php
@@ -34,6 +34,14 @@
  *     $ wp config get table_prefix
  *     wp_
  *
+ *     # Add a new constant to the wp-config.php file.
+ *     $ wp config add WP_DEBUG true --raw
+ *     Success: Added the constant 'WP_DEBUG' to the 'wp-config.php' file with the raw value 'true'.
+ *
+ *     # Update or add a constant to the wp-config.php file.
+ *     $ wp config update WP_DEBUG false --raw
+ *     Success: Updated the constant 'WP_DEBUG' in the 'wp-config.php' file with the raw value 'false'.
+ *
  *     # Set the WP_DEBUG constant to true.
  *     $ wp config set WP_DEBUG true --raw
  *     Success: Updated the constant 'WP_DEBUG' in the 'wp-config.php' file with the raw value 'true'.
@@ -739,6 +747,255 @@ public function set( $args, $assoc_args ) {
 		WP_CLI::success( $message );
 	}
 
+	/**
+	 * Adds a new constant or variable to the wp-config.php file.
+	 *
+	 * ## OPTIONS
+	 *
+	 * <name>
+	 * : Name of the wp-config.php constant or variable.
+	 *
+	 * <value>
+	 * : Value to set the wp-config.php constant or variable to.
+	 *
+	 * [--raw]
+	 * : Place the value into the wp-config.php file as is, instead of as a quoted string.
+	 *
+	 * [--anchor=<anchor>]
+	 * : Anchor string where additions of new values are anchored around.
+	 * Defaults to "/* That's all, stop editing!".
+	 * The special case "EOF" string uses the end of the file as the anchor.
+	 *
+	 * [--placement=<placement>]
+	 * : Where to place the new values in relation to the anchor string.
+	 * ---
+	 * default: 'before'
+	 * options:
+	 *   - before
+	 *   - after
+	 * ---
+	 *
+	 * [--separator=<separator>]
+	 * : Separator string to put between an added value and its anchor string.
+	 * The following escape sequences will be recognized and properly interpreted: '\n' => newline, '\r' => carriage return, '\t' => tab.
+	 * Defaults to a single EOL ("\n" on *nix and "\r\n" on Windows).
+	 *
+	 * [--type=<type>]
+	 * : Type of the config value to add. Defaults to 'constant'.
+	 * ---
+	 * default: constant
+	 * options:
+	 *   - constant
+	 *   - variable
+	 * ---
+	 *
+	 * [--config-file=<path>]
+	 * : Specify the file path to the config file to be modified. Defaults to the root of the
+	 * WordPress installation and the filename "wp-config.php".
+	 *
+	 * ## EXAMPLES
+	 *
+	 *     # Add the WP_DEBUG constant to the wp-config.php file.
+	 *     $ wp config add WP_DEBUG true --raw
+	 *     Success: Added the constant 'WP_DEBUG' to the 'wp-config.php' file with the raw value 'true'.
+	 *
+	 * @when before_wp_load
+	 */
+	public function add( $args, $assoc_args ) {
+		$path                 = $this->get_config_path( $assoc_args );
+		$wp_config_file_name  = basename( $path );
+		list( $name, $value ) = $args;
+
+		/**
+		 * @var string $type
+		 */
+		$type = Utils\get_flag_value( $assoc_args, 'type', 'constant' );
+
+		$options = [];
+
+		$option_flags = [
+			'raw'       => false,
+			'anchor'    => null,
+			'placement' => null,
+			'separator' => null,
+		];
+
+		foreach ( $option_flags as $option => $default ) {
+			/**
+			 * @var string|null $option_value
+			 */
+			$option_value = Utils\get_flag_value( $assoc_args, $option, $default );
+			if ( null !== $option_value ) {
+				/**
+				 * @var array<string, string> $options
+				 */
+				$options[ $option ] = $option_value;
+				if ( 'separator' === $option ) {
+					$options['separator'] = $this->parse_separator( $options['separator'] );
+				}
+			}
+		}
+
+		// add command always adds, so we set the 'add' option to true
+		$options['add'] = true;
+
+		try {
+			$config_transformer = new WPConfigTransformer( $path );
+
+			// Check if the constant/variable already exists
+			if ( $config_transformer->exists( $type, $name ) ) {
+				WP_CLI::error( "The {$type} '{$name}' already exists in the '{$wp_config_file_name}' file." );
+			}
+
+			$config_transformer->update( $type, $name, $value, $options );
+
+		} catch ( Exception $exception ) {
+			WP_CLI::error( "Could not process the '{$wp_config_file_name}' transformation.\nReason: {$exception->getMessage()}" );
+		}
+
+		$raw     = $options['raw'] ? 'raw ' : '';
+		$message = "Added the {$type} '{$name}' to the '{$wp_config_file_name}' file with the {$raw}value '{$value}'.";
+
+		WP_CLI::success( $message );
+	}
+
+	/**
+	 * Updates or adds a constant or variable in the wp-config.php file.
+	 *
+	 * This command will add the constant or variable if it doesn't exist, or update it if it does.
+	 *
+	 * ## OPTIONS
+	 *
+	 * <name>
+	 * : Name of the wp-config.php constant or variable.
+	 *
+	 * <value>
+	 * : Value to set the wp-config.php constant or variable to.
+	 *
+	 * [--raw]
+	 * : Place the value into the wp-config.php file as is, instead of as a quoted string.
+	 *
+	 * [--anchor=<anchor>]
+	 * : Anchor string where additions of new values are anchored around.
+	 * Defaults to "/* That's all, stop editing!".
+	 * The special case "EOF" string uses the end of the file as the anchor.
+	 *
+	 * [--placement=<placement>]
+	 * : Where to place the new values in relation to the anchor string.
+	 * ---
+	 * default: 'before'
+	 * options:
+	 *   - before
+	 *   - after
+	 * ---
+	 *
+	 * [--separator=<separator>]
+	 * : Separator string to put between an added value and its anchor string.
+	 * The following escape sequences will be recognized and properly interpreted: '\n' => newline, '\r' => carriage return, '\t' => tab.
+	 * Defaults to a single EOL ("\n" on *nix and "\r\n" on Windows).
+	 *
+	 * [--type=<type>]
+	 * : Type of the config value to update. Defaults to 'all'.
+	 * ---
+	 * default: all
+	 * options:
+	 *   - constant
+	 *   - variable
+	 *   - all
+	 * ---
+	 *
+	 * [--config-file=<path>]
+	 * : Specify the file path to the config file to be modified. Defaults to the root of the
+	 * WordPress installation and the filename "wp-config.php".
+	 *
+	 * ## EXAMPLES
+	 *
+	 *     # Update or add the WP_DEBUG constant to the wp-config.php file.
+	 *     $ wp config update WP_DEBUG true --raw
+	 *     Success: Updated the constant 'WP_DEBUG' in the 'wp-config.php' file with the raw value 'true'.
+	 *
+	 * @when before_wp_load
+	 */
+	public function update( $args, $assoc_args ) {
+		$path                 = $this->get_config_path( $assoc_args );
+		$wp_config_file_name  = basename( $path );
+		list( $name, $value ) = $args;
+
+		/**
+		 * @var string $type
+		 */
+		$type = Utils\get_flag_value( $assoc_args, 'type', 'all' );
+
+		$options = [];
+
+		$option_flags = [
+			'raw'       => false,
+			'anchor'    => null,
+			'placement' => null,
+			'separator' => null,
+		];
+
+		foreach ( $option_flags as $option => $default ) {
+			/**
+			 * @var string|null $option_value
+			 */
+			$option_value = Utils\get_flag_value( $assoc_args, $option, $default );
+			if ( null !== $option_value ) {
+				/**
+				 * @var array<string, string> $options
+				 */
+				$options[ $option ] = $option_value;
+				if ( 'separator' === $option ) {
+					$options['separator'] = $this->parse_separator( $options['separator'] );
+				}
+			}
+		}
+
+		// update command always adds if not exists, so we set the 'add' option to true
+		$options['add'] = true;
+
+		$adding = false;
+		try {
+			$config_transformer = new WPConfigTransformer( $path );
+
+			switch ( $type ) {
+				case 'all':
+					$has_constant = $config_transformer->exists( 'constant', $name );
+					$has_variable = $config_transformer->exists( 'variable', $name );
+					if ( $has_constant && $has_variable ) {
+						WP_CLI::error( "Found both a constant and a variable '{$name}' in the '{$wp_config_file_name}' file. Use --type=<type> to disambiguate." );
+					}
+					if ( ! $has_constant && ! $has_variable ) {
+						$type   = 'constant';
+						$adding = true;
+					} else {
+						$type = $has_constant ? 'constant' : 'variable';
+					}
+					break;
+				case 'constant':
+				case 'variable':
+					if ( ! $config_transformer->exists( $type, $name ) ) {
+						$adding = true;
+					}
+					break;
+			}
+
+			$config_transformer->update( $type, $name, $value, $options );
+
+		} catch ( Exception $exception ) {
+			WP_CLI::error( "Could not process the '{$wp_config_file_name}' transformation.\nReason: {$exception->getMessage()}" );
+		}
+
+		$raw = $options['raw'] ? 'raw ' : '';
+		if ( $adding ) {
+			$message = "Added the {$type} '{$name}' to the '{$wp_config_file_name}' file with the {$raw}value '{$value}'.";
+		} else {
+			$message = "Updated the {$type} '{$name}' in the '{$wp_config_file_name}' file with the {$raw}value '{$value}'.";
+		}
+
+		WP_CLI::success( $message );
+	}
+
 	/**
 	 * Deletes a specific constant or variable from the wp-config.php file.
 	 *