diff --git a/playground/commands/deploy.ts b/playground/commands/deploy.ts
index fe53689..e6277af 100644
--- a/playground/commands/deploy.ts
+++ b/playground/commands/deploy.ts
@@ -52,6 +52,14 @@ export default defineCommand({
       description: "Deployment provider",
       valueHint: "foo|bar|baz|qux",
     },
+    port: {
+      type: "string",
+      description: "Port number to listen on",
+      required: true,
+      validate(value) {
+        return Number(value) >= 1 && Number(value) <= 65_536 ? true : "Port number must be greater than 1 and less than 65536"
+      },
+    }
   },
   run({ args }) {
     consola.log("Build");
diff --git a/src/args.ts b/src/args.ts
index ab09e84..4be5f42 100644
--- a/src/args.ts
+++ b/src/args.ts
@@ -116,3 +116,20 @@ export function resolveArgs(argsDef: ArgsDef): Arg[] {
   }
   return args;
 }
+
+export function resolveArgsValidate(
+  parsedArgs: ParsedArgs,
+  argsDef: ArgsDef,
+): string | undefined {
+  for (const [name, argDef] of Object.entries(argsDef || {})) {
+    const value = parsedArgs[name] as never;
+    if (argDef.validate) {
+      const word = argDef.validate(value) || "";
+      if (argDef.required && typeof word === "string") {
+        return (
+          `Argument validation failed: ${name}` + (word ? ` - ${word}` : "")
+        );
+      }
+    }
+  }
+}
diff --git a/src/command.ts b/src/command.ts
index 4878197..df35640 100644
--- a/src/command.ts
+++ b/src/command.ts
@@ -1,6 +1,6 @@
 import type { CommandContext, CommandDef, ArgsDef } from "./types";
 import { CLIError, resolveValue } from "./_utils";
-import { parseArgs } from "./args";
+import { parseArgs, resolveArgsValidate } from "./args";
 
 export function defineCommand<const T extends ArgsDef = ArgsDef>(
   def: CommandDef<T>,
@@ -59,7 +59,10 @@ export async function runCommand<T extends ArgsDef = ArgsDef>(
         throw new CLIError(`No command specified.`, "E_NO_COMMAND");
       }
     }
-
+    const word = resolveArgsValidate(parsedArgs, cmdArgs);
+    if (word) {
+      throw new CLIError(word, "E_VALIDATE_FAILED");
+    }
     // Handle main command
     if (typeof cmd.run === "function") {
       result = await cmd.run(context);
diff --git a/src/types.ts b/src/types.ts
index e9892f6..ae88cc6 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -17,6 +17,7 @@ export type _ArgDef<T extends ArgType, VT extends boolean | number | string> = {
   alias?: string | string[];
   default?: VT;
   required?: boolean;
+  validate?: (value: ParsedArgType<T>) => boolean | string;
   options?: (string | number)[];
 };
 
@@ -100,6 +101,14 @@ export type ParsedArgs<T extends ArgsDef = ArgsDef> = RawArgs &
   { [K in keyof T as T[K] extends { alias: string[] } ? T[K]["alias"][number] : never]: ParsedArg<T[K]> } &
   Record<string, string | number | boolean | string[]>;
 
+export type ParsedArgType<T extends ArgType = ArgType> = T extends "positional"
+  ? string
+  : T extends "string"
+    ? string
+    : T extends "boolean"
+      ? boolean
+      : string | boolean;
+
 // ----- Command -----
 
 // Command: Shared