diff --git a/CHANGELOG.md b/CHANGELOG.md
index dd2b3c2879f9..5b7d8ca4aa29 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Reintroduce `max-w-screen-*` utilities that read from the `--breakpoint` namespace as deprecated utilities ([#15013](https://github.com/tailwindlabs/tailwindcss/pull/15013))
 - Support using CSS variables as arbitrary values without `var(…)` by using parentheses instead of square brackets (e.g. `bg-(--my-color)`) ([#15020](https://github.com/tailwindlabs/tailwindcss/pull/15020))
+- Add new `in-*` variant ([#15025](https://github.com/tailwindlabs/tailwindcss/pull/15025))
 - _Upgrade (experimental)_: Migrate `[&>*]` to the `*` variant ([#15022](https://github.com/tailwindlabs/tailwindcss/pull/15022))
 - _Upgrade (experimental)_: Migrate `[&_*]` to the `**` variant ([#15022](https://github.com/tailwindlabs/tailwindcss/pull/15022))
 
diff --git a/packages/@tailwindcss-upgrade/src/template/codemods/modernize-arbitrary-values.test.ts b/packages/@tailwindcss-upgrade/src/template/codemods/modernize-arbitrary-values.test.ts
index f5a2c3d7e6d4..51d26a107eab 100644
--- a/packages/@tailwindcss-upgrade/src/template/codemods/modernize-arbitrary-values.test.ts
+++ b/packages/@tailwindcss-upgrade/src/template/codemods/modernize-arbitrary-values.test.ts
@@ -18,6 +18,11 @@ test.each([
   ['[&:first-child]:flex', 'first:flex'],
   ['[&:not(:first-child)]:flex', 'not-first:flex'],
 
+  // in-* variants
+  ['[p_&]:flex', 'in-[p]:flex'],
+  ['[.foo_&]:flex', 'in-[.foo]:flex'],
+  ['[[data-visible]_&]:flex', 'in-data-visible:flex'],
+
   // nth-child
   ['[&:nth-child(2)]:flex', 'nth-2:flex'],
   ['[&:not(:nth-child(2))]:flex', 'not-nth-2:flex'],
diff --git a/packages/@tailwindcss-upgrade/src/template/codemods/modernize-arbitrary-values.ts b/packages/@tailwindcss-upgrade/src/template/codemods/modernize-arbitrary-values.ts
index 7bcd252eb797..3efa3f5ddd38 100644
--- a/packages/@tailwindcss-upgrade/src/template/codemods/modernize-arbitrary-values.ts
+++ b/packages/@tailwindcss-upgrade/src/template/codemods/modernize-arbitrary-values.ts
@@ -15,6 +15,19 @@ export function modernizeArbitraryValues(
     let changed = false
 
     for (let [variant, parent] of variants(clone)) {
+      // Forward modifier from the root to the compound variant
+      if (
+        variant.kind === 'compound' &&
+        (variant.root === 'has' || variant.root === 'not' || variant.root === 'in')
+      ) {
+        if (variant.modifier !== null) {
+          if ('modifier' in variant.variant) {
+            variant.variant.modifier = variant.modifier
+            variant.modifier = null
+          }
+        }
+      }
+
       // Expecting an arbitrary variant
       if (variant.kind !== 'arbitrary') continue
 
@@ -98,6 +111,61 @@ export function modernizeArbitraryValues(
         prefixedVariant = designSystem.parseVariant('**')
       }
 
+      // Handling a child/parent combinator. E.g.: `[[data-visible]_&]` => `in-data-visible`
+      if (
+        // Only top-level, so `has-[&_[data-visible]]` is not supported
+        parent === null &&
+        // [[data-visible]___&]:flex
+        //  ^^^^^^^^^^^^^^ ^ ^
+        ast.nodes[0].length === 3 &&
+        ast.nodes[0].nodes[0].type === 'attribute' &&
+        ast.nodes[0].nodes[1].type === 'combinator' &&
+        ast.nodes[0].nodes[1].value === ' ' &&
+        ast.nodes[0].nodes[2].type === 'nesting' &&
+        ast.nodes[0].nodes[2].value === '&'
+      ) {
+        ast.nodes[0].nodes = [ast.nodes[0].nodes[0]]
+        changed = true
+        // When handling a compound like `in-[[data-visible]]`, we will first
+        // handle `[[data-visible]]`, then the parent `in-*` part. This means
+        // that we can convert `[[data-visible]_&]` to `in-[[data-visible]]`.
+        //
+        // Later this gets converted to `in-data-visible`.
+        Object.assign(variant, designSystem.parseVariant(`in-[${ast.toString()}]`))
+        continue
+      }
+
+      // `in-*` variant
+      if (
+        // Only top-level, so `has-[p_&]` is not supported
+        parent === null &&
+        // `[data-*]` and `[aria-*]` are handled separately
+        !(
+          ast.nodes[0].nodes[0].type === 'attribute' &&
+          (ast.nodes[0].nodes[0].attribute.startsWith('data-') ||
+            ast.nodes[0].nodes[0].attribute.startsWith('aria-'))
+        ) &&
+        // [.foo___&]:flex
+        //  ^^^^ ^ ^
+        ast.nodes[0].nodes.at(-1)?.type === 'nesting'
+      ) {
+        let selector = ast.nodes[0]
+        let nodes = selector.nodes
+
+        nodes.pop() // Remove the last node `&`
+
+        // Remove trailing whitespace
+        let last = nodes.at(-1)
+        while (last?.type === 'combinator' && last.value === ' ') {
+          nodes.pop()
+          last = nodes.at(-1)
+        }
+
+        changed = true
+        Object.assign(variant, designSystem.parseVariant(`in-[${selector.toString().trim()}]`))
+        continue
+      }
+
       // Filter out `&`. E.g.: `&[data-foo]` => `[data-foo]`
       let selectorNodes = ast.nodes[0].filter((node) => node.type !== 'nesting')
 
diff --git a/packages/tailwindcss/src/__snapshots__/intellisense.test.ts.snap b/packages/tailwindcss/src/__snapshots__/intellisense.test.ts.snap
index a7a60326cd9e..aa80fc0376e7 100644
--- a/packages/tailwindcss/src/__snapshots__/intellisense.test.ts.snap
+++ b/packages/tailwindcss/src/__snapshots__/intellisense.test.ts.snap
@@ -7554,6 +7554,7 @@ exports[`getVariants 1`] = `
       "enabled",
       "disabled",
       "inert",
+      "in",
       "has",
       "aria",
       "data",
@@ -7622,6 +7623,7 @@ exports[`getVariants 1`] = `
       "enabled",
       "disabled",
       "inert",
+      "in",
       "has",
       "aria",
       "data",
@@ -7674,6 +7676,7 @@ exports[`getVariants 1`] = `
       "enabled",
       "disabled",
       "inert",
+      "in",
       "has",
       "aria",
       "data",
@@ -7972,6 +7975,59 @@ exports[`getVariants 1`] = `
     "selectors": [Function],
     "values": [],
   },
+  {
+    "hasDash": true,
+    "isArbitrary": true,
+    "name": "in",
+    "selectors": [Function],
+    "values": [
+      "not",
+      "group",
+      "peer",
+      "first",
+      "last",
+      "only",
+      "odd",
+      "even",
+      "first-of-type",
+      "last-of-type",
+      "only-of-type",
+      "visited",
+      "target",
+      "open",
+      "default",
+      "checked",
+      "indeterminate",
+      "placeholder-shown",
+      "autofill",
+      "optional",
+      "required",
+      "valid",
+      "invalid",
+      "in-range",
+      "out-of-range",
+      "read-only",
+      "empty",
+      "focus-within",
+      "hover",
+      "focus",
+      "focus-visible",
+      "active",
+      "enabled",
+      "disabled",
+      "inert",
+      "in",
+      "has",
+      "aria",
+      "data",
+      "nth",
+      "nth-last",
+      "nth-of-type",
+      "nth-last-of-type",
+      "ltr",
+      "rtl",
+    ],
+  },
   {
     "hasDash": true,
     "isArbitrary": true,
@@ -8013,6 +8069,7 @@ exports[`getVariants 1`] = `
       "enabled",
       "disabled",
       "inert",
+      "in",
       "has",
       "aria",
       "data",
diff --git a/packages/tailwindcss/src/variants.test.ts b/packages/tailwindcss/src/variants.test.ts
index 54cbe2dbf5d0..c6510fc26fdc 100644
--- a/packages/tailwindcss/src/variants.test.ts
+++ b/packages/tailwindcss/src/variants.test.ts
@@ -1693,6 +1693,23 @@ test('not', async () => {
   ).toEqual('')
 })
 
+test('in', async () => {
+  expect(
+    await run([
+      'in-[p]:flex',
+      'in-[.group]:flex',
+      'not-in-[p]:flex',
+      'not-in-[.group]:flex',
+      'in-data-visible:flex',
+    ]),
+  ).toMatchInlineSnapshot(`
+    ".not-in-\\[\\.group\\]\\:flex:not(:where(.group) *), .not-in-\\[p\\]\\:flex:not(:where(:is(p)) *), :where([data-visible]) .in-data-visible\\:flex, :where(.group) .in-\\[\\.group\\]\\:flex, :where(:is(p)) .in-\\[p\\]\\:flex {
+      display: flex;
+    }"
+  `)
+  expect(await run(['in-p:flex', 'in-foo-bar:flex'])).toEqual('')
+})
+
 test('has', async () => {
   expect(
     await compileCss(
diff --git a/packages/tailwindcss/src/variants.ts b/packages/tailwindcss/src/variants.ts
index 2a636f20831c..a88f3d50660f 100644
--- a/packages/tailwindcss/src/variants.ts
+++ b/packages/tailwindcss/src/variants.ts
@@ -711,6 +711,41 @@ export function createVariants(theme: Theme): Variants {
 
   staticVariant('inert', ['&:is([inert], [inert] *)'])
 
+  variants.compound('in', Compounds.StyleRules, (ruleNode, variant) => {
+    if (variant.modifier) return null
+
+    let didApply = false
+
+    walk([ruleNode], (node, { path }) => {
+      if (node.kind !== 'rule') return WalkAction.Continue
+
+      // Throw out any candidates with variants using nested style rules
+      for (let parent of path.slice(0, -1)) {
+        if (parent.kind !== 'rule') continue
+
+        didApply = false
+        return WalkAction.Stop
+      }
+
+      // Replace `&` in target variant with `*`, so variants like `&:hover`
+      // become `:where(*:hover) &`. The `*` will often be optimized away.
+      node.selector = `:where(${node.selector.replaceAll('&', '*')}) &`
+
+      // Track that the variant was actually applied
+      didApply = true
+    })
+
+    // If the node wasn't modified, this variant is not compatible with
+    // `in-*` so discard the candidate.
+    if (!didApply) return null
+  })
+
+  variants.suggest('in', () => {
+    return Array.from(variants.keys()).filter((name) => {
+      return variants.compoundsWith('in', name)
+    })
+  })
+
   variants.compound('has', Compounds.StyleRules, (ruleNode, variant) => {
     if (variant.modifier) return null