diff --git a/.changeset/new-donkeys-attend.md b/.changeset/new-donkeys-attend.md
new file mode 100644
index 00000000000..ec5401959fa
--- /dev/null
+++ b/.changeset/new-donkeys-attend.md
@@ -0,0 +1,5 @@
+---
+"@primer/react": minor
+---
+
+Migrate DialogV1 to CSS Modules
diff --git a/packages/react/src/DialogV1/Dialog.module.css b/packages/react/src/DialogV1/Dialog.module.css
new file mode 100644
index 00000000000..dba07129c37
--- /dev/null
+++ b/packages/react/src/DialogV1/Dialog.module.css
@@ -0,0 +1,67 @@
+.Overlay {
+  &::before {
+    position: fixed;
+    top: 0;
+    right: 0;
+    bottom: 0;
+    left: 0;
+    z-index: 99;
+    display: block;
+    cursor: default;
+    content: ' ';
+    background: var(--overlay-backdrop-bgColor);
+  }
+}
+
+.CloseIcon {
+  position: absolute;
+  top: var(--base-size-8);
+  right: var(--base-size-16);
+}
+
+.Dialog {
+  position: fixed;
+  top: 0;
+  left: 50%;
+  z-index: 999;
+  max-height: 80vh;
+  margin: 10vh auto;
+  background-color: var(--bgColor-default);
+  border-radius: var(--borderRadius-medium);
+  outline: none;
+  box-shadow: var(--shadow-floating-large);
+  transform: translateX(-50%);
+
+  &:where([data-width='default']) {
+    /* stylelint-disable-next-line primer/responsive-widths */
+    width: 440px;
+  }
+
+  &:where([data-width='narrow']) {
+    width: 320px;
+  }
+
+  &:where([data-width='wide']) {
+    /* stylelint-disable-next-line primer/responsive-widths */
+    width: 640px;
+  }
+
+  @media screen and (max-width: 750px) {
+    width: 100dvw;
+    height: 100dvh;
+    margin: 0;
+    border-radius: 0;
+  }
+}
+
+.Header {
+  display: flex;
+  padding: var(--base-size-16);
+  background: var(--bgColor-muted);
+  border-bottom: var(--borderWidth-thin) solid var(--borderColor-default);
+  border-radius: var(--borderRadius-medium) var(--borderRadius-medium) 0 0;
+
+  @media screen and (max-width: 750px) {
+    border-radius: 0;
+  }
+}
diff --git a/packages/react/src/DialogV1/Dialog.test.tsx b/packages/react/src/DialogV1/Dialog.test.tsx
index 7df87bd2c75..d4023e6365c 100644
--- a/packages/react/src/DialogV1/Dialog.test.tsx
+++ b/packages/react/src/DialogV1/Dialog.test.tsx
@@ -4,6 +4,7 @@ import {Dialog} from '../DialogV1'
 import {render as HTMLRender, fireEvent} from '@testing-library/react'
 import axe from 'axe-core'
 import {behavesAsComponent} from '../utils/testing'
+import {FeatureFlags} from '../FeatureFlags'
 
 /* Dialog Version 1*/
 
@@ -113,6 +114,25 @@ describe('Dialog', () => {
     behavesAsComponent({Component: Dialog.Header})
   })
 
+  it('should support `className` on the Dialog element', () => {
+    const Element = () => <Dialog isOpen className={'test-class-name'} />
+    const FeatureFlagElement = () => {
+      return (
+        <FeatureFlags
+          flags={{
+            primer_react_css_modules_team: true,
+            primer_react_css_modules_staff: true,
+            primer_react_css_modules_ga: true,
+          }}
+        >
+          <Element />
+        </FeatureFlags>
+      )
+    }
+    expect(HTMLRender(<Element />).container.children[1]).toHaveClass('test-class-name')
+    expect(HTMLRender(<FeatureFlagElement />).container.children[1]).toHaveClass('test-class-name')
+  })
+
   it('should have no axe violations', async () => {
     const spy = jest.spyOn(console, 'warn').mockImplementation()
     const {container} = HTMLRender(comp)
diff --git a/packages/react/src/DialogV1/Dialog.tsx b/packages/react/src/DialogV1/Dialog.tsx
index 3e565ba8d3d..eababe28cb7 100644
--- a/packages/react/src/DialogV1/Dialog.tsx
+++ b/packages/react/src/DialogV1/Dialog.tsx
@@ -10,6 +10,12 @@ import Text from '../Text'
 import type {ComponentProps} from '../utils/types'
 import {useRefObjectAsForwardedRef} from '../hooks/useRefObjectAsForwardedRef'
 import {XIcon} from '@primer/octicons-react'
+import {toggleStyledComponent} from '../internal/utils/toggleStyledComponent'
+import {useFeatureFlag} from '../FeatureFlags'
+import {clsx} from 'clsx'
+import classes from './Dialog.module.css'
+
+const CSS_MODULES_FEATURE_FLAG = 'primer_react_css_modules_team'
 
 // Dialog v1
 const noop = () => null
@@ -19,41 +25,50 @@ type StyledDialogBaseProps = {
   wide?: boolean
 } & SxProp
 
-const DialogBase = styled.div<StyledDialogBaseProps>`
-  box-shadow: ${get('shadows.shadow.large')};
-  border-radius: ${get('radii.2')};
-  position: fixed;
-  top: 0;
-  left: 50%;
-  transform: translateX(-50%);
-  max-height: 80vh;
-  z-index: 999;
-  margin: 10vh auto;
-  background-color: ${get('colors.canvas.default')};
-  width: ${props => (props.narrow ? '320px' : props.wide ? '640px' : '440px')};
-  outline: none;
-
-  @media screen and (max-width: 750px) {
-    width: 100dvw;
-    margin: 0;
-    border-radius: 0;
-    height: 100dvh;
-  }
+const DialogBase = toggleStyledComponent(
+  CSS_MODULES_FEATURE_FLAG,
+  'div',
+  styled.div<StyledDialogBaseProps>`
+    box-shadow: ${get('shadows.shadow.large')};
+    border-radius: ${get('radii.2')};
+    position: fixed;
+    top: 0;
+    left: 50%;
+    transform: translateX(-50%);
+    max-height: 80vh;
+    z-index: 999;
+    margin: 10vh auto;
+    background-color: ${get('colors.canvas.default')};
+    width: ${props => (props.narrow ? '320px' : props.wide ? '640px' : '440px')};
+    outline: none;
+
+    @media screen and (max-width: 750px) {
+      width: 100dvw;
+      margin: 0;
+      border-radius: 0;
+      height: 100dvh;
+    }
 
-  ${sx};
-`
+    ${sx};
+  `,
+)
 
-const DialogHeaderBase = styled(Box)<SxProp>`
-  border-radius: ${get('radii.2')} ${get('radii.2')} 0px 0px;
-  border-bottom: 1px solid ${get('colors.border.default')};
-  display: flex;
+const DialogHeaderBase = toggleStyledComponent(
+  CSS_MODULES_FEATURE_FLAG,
+  'div',
+  styled(Box)<SxProp>`
+    border-radius: ${get('radii.2')} ${get('radii.2')} 0px 0px;
+    border-bottom: 1px solid ${get('colors.border.default')};
+    display: flex;
 
-  @media screen and (max-width: 750px) {
-    border-radius: 0px;
-  }
+    @media screen and (max-width: 750px) {
+      border-radius: 0px;
+    }
+
+    ${sx};
+  `,
+)
 
-  ${sx};
-`
 export type DialogHeaderProps = ComponentProps<typeof DialogHeaderBase>
 
 function DialogHeader({theme, children, backgroundColor = 'canvas.subtle', ...rest}: DialogHeaderProps) {
@@ -65,28 +80,40 @@ function DialogHeader({theme, children, backgroundColor = 'canvas.subtle', ...re
     )
   }
 
+  const enabled = useFeatureFlag(CSS_MODULES_FEATURE_FLAG)
+
   return (
-    <DialogHeaderBase theme={theme} p={3} backgroundColor={backgroundColor} {...rest}>
+    <DialogHeaderBase
+      theme={theme}
+      p={3}
+      backgroundColor={backgroundColor}
+      {...rest}
+      className={enabled ? classes.Header : undefined}
+    >
       {children}
     </DialogHeaderBase>
   )
 }
 
-const Overlay = styled.span`
-  &:before {
-    position: fixed;
-    top: 0;
-    right: 0;
-    bottom: 0;
-    left: 0;
-    display: block;
-    cursor: default;
-    content: ' ';
-    background: transparent;
-    z-index: 99;
-    background: ${get('colors.primer.canvas.backdrop')};
-  }
-`
+const Overlay = toggleStyledComponent(
+  CSS_MODULES_FEATURE_FLAG,
+  'span',
+  styled.span`
+    &:before {
+      position: fixed;
+      top: 0;
+      right: 0;
+      bottom: 0;
+      left: 0;
+      display: block;
+      cursor: default;
+      content: ' ';
+      background: transparent;
+      z-index: 99;
+      background: ${get('colors.primer.canvas.backdrop')};
+    }
+  `,
+)
 
 type InternalDialogProps = {
   isOpen?: boolean
@@ -96,7 +123,7 @@ type InternalDialogProps = {
 } & ComponentProps<typeof DialogBase>
 
 const Dialog = forwardRef<HTMLDivElement, InternalDialogProps>(
-  ({children, onDismiss = noop, isOpen, initialFocusRef, returnFocusRef, ...props}, forwardedRef) => {
+  ({children, onDismiss = noop, isOpen, initialFocusRef, returnFocusRef, className, ...props}, forwardedRef) => {
     const overlayRef = useRef(null)
     const modalRef = useRef<HTMLDivElement>(null)
     useRefObjectAsForwardedRef(forwardedRef, modalRef)
@@ -118,17 +145,33 @@ const Dialog = forwardRef<HTMLDivElement, InternalDialogProps>(
       returnFocusRef,
       overlayRef,
     })
+
+    const enabled = useFeatureFlag(CSS_MODULES_FEATURE_FLAG)
+
+    const iconStyles = enabled
+      ? {className: classes.CloseIcon}
+      : {sx: {position: 'absolute', top: '8px', right: '16px'}}
+
     return isOpen ? (
       <>
-        <Overlay ref={overlayRef} />
-        <DialogBase tabIndex={-1} ref={modalRef} role="dialog" aria-modal="true" {...props} {...getDialogProps()}>
+        <Overlay className={enabled ? classes.Overlay : undefined} ref={overlayRef} />
+        <DialogBase
+          tabIndex={-1}
+          ref={modalRef}
+          role="dialog"
+          aria-modal="true"
+          {...props}
+          {...getDialogProps()}
+          className={clsx({[classes.Dialog]: enabled}, className)}
+          data-width={props.wide ? 'wide' : props.narrow ? 'narrow' : 'default'}
+        >
           <IconButton
             icon={XIcon}
             ref={closeButtonRef}
             onClick={onCloseClick}
-            sx={{position: 'absolute', top: '8px', right: '16px'}}
             aria-label="Close"
             variant="invisible"
+            {...iconStyles}
           />
           {children}
         </DialogBase>
diff --git a/packages/react/src/DialogV1/Dialog.types.test.tsx b/packages/react/src/DialogV1/Dialog.types.test.tsx
index 010f84f6fed..8c31a1a8768 100644
--- a/packages/react/src/DialogV1/Dialog.types.test.tsx
+++ b/packages/react/src/DialogV1/Dialog.types.test.tsx
@@ -4,8 +4,3 @@ import {Dialog} from '../DialogV1'
 export function shouldAcceptCallWithNoProps() {
   return <Dialog />
 }
-
-export function shouldNotAcceptSystemProps() {
-  // @ts-expect-error system props should not be accepted
-  return <Dialog backgroundColor="thistle" />
-}