diff --git a/CHANGELOG.md b/CHANGELOG.md
index b049c4cf7e68..1c2c86eb22e0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -33,6 +33,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Pass options when using `addComponents` and `matchComponents` ([#14590](https://github.com/tailwindlabs/tailwindcss/pull/14590))
 - Ensure `boxShadow` and `animation` theme keys in JS config files are accessible under `--shadow-*` and `--animate-*` using the `theme()` function ([#14642](https://github.com/tailwindlabs/tailwindcss/pull/14642))
 - Ensure all theme keys with new names are also accessible under their old names when using the `theme()` function with the legacy dot notation syntax ([#14642](https://github.com/tailwindlabs/tailwindcss/pull/14642))
+- Ensure `var(…)` can be used as the opacity value inside the `theme([path] / [modifier])` function ([#14653](https://github.com/tailwindlabs/tailwindcss/pull/14653))
 - _Upgrade (experimental)_: Ensure CSS before a layer stays unlayered when running codemods ([#14596](https://github.com/tailwindlabs/tailwindcss/pull/14596))
 - _Upgrade (experimental)_: Resolve issues where some prefixed candidates were not properly migrated ([#14600](https://github.com/tailwindlabs/tailwindcss/pull/14600))
 
diff --git a/packages/tailwindcss/src/css-functions.test.ts b/packages/tailwindcss/src/css-functions.test.ts
index cb54f3406972..a2df1994ff70 100644
--- a/packages/tailwindcss/src/css-functions.test.ts
+++ b/packages/tailwindcss/src/css-functions.test.ts
@@ -136,6 +136,27 @@ describe('theme function', () => {
         `)
       })
 
+      test('theme(colors.red.500/75%)', async () => {
+        expect(
+          await compileCss(css`
+            @theme {
+              --color-red-500: #f00;
+            }
+            .red {
+              color: theme(colors.red.500/75%);
+            }
+          `),
+        ).toMatchInlineSnapshot(`
+          ":root {
+            --color-red-500: red;
+          }
+
+          .red {
+            color: #ff0000bf;
+          }"
+        `)
+      })
+
       test('theme(colors.red.500 / 75%)', async () => {
         expect(
           await compileCss(css`
@@ -178,6 +199,49 @@ describe('theme function', () => {
         `)
       })
 
+      test('theme(colors.red.500/var(--opacity))', async () => {
+        expect(
+          await compileCss(css`
+            @theme {
+              --color-red-500: #f00;
+            }
+            .red {
+              color: theme(colors.red.500/var(--opacity));
+            }
+          `),
+        ).toMatchInlineSnapshot(`
+          ":root {
+            --color-red-500: red;
+          }
+
+          .red {
+            color: color-mix(in srgb, red calc(var(--opacity) * 100%), transparent);
+          }"
+        `)
+      })
+
+      test('theme(colors.red.500/var(--opacity,50%))', async () => {
+        expect(
+          await compileCss(css`
+            @theme {
+              --color-red-500: #f00;
+            }
+            .red {
+              /* prettier-ignore */
+              color: theme(colors.red.500/var(--opacity,50%));
+            }
+          `),
+        ).toMatchInlineSnapshot(`
+          ":root {
+            --color-red-500: red;
+          }
+
+          .red {
+            color: color-mix(in srgb, red calc(var(--opacity, 50%) * 100%), transparent);
+          }"
+        `)
+      })
+
       test('theme(spacing.12)', async () => {
         expect(
           await compileCss(css`
diff --git a/packages/tailwindcss/src/css-functions.ts b/packages/tailwindcss/src/css-functions.ts
index 5decddca14d4..80cbd8fbe15c 100644
--- a/packages/tailwindcss/src/css-functions.ts
+++ b/packages/tailwindcss/src/css-functions.ts
@@ -68,7 +68,7 @@ export function substituteFunctionsInValue(
         if (node.nodes[i].value.includes(',')) {
           break
         }
-        path += node.nodes[i].value
+        path += ValueParser.toCss([node.nodes[i]])
         skipUntilIndex = i + 1
       }
 
diff --git a/packages/tailwindcss/src/value-parser.test.ts b/packages/tailwindcss/src/value-parser.test.ts
index b90033fbc0c3..2814b2e9efa1 100644
--- a/packages/tailwindcss/src/value-parser.test.ts
+++ b/packages/tailwindcss/src/value-parser.test.ts
@@ -66,6 +66,20 @@ describe('parse', () => {
     ])
   })
 
+  it('should parse a function with nested arguments separated by `/`', () => {
+    expect(parse('theme(colors.red.500/var(--opacity))')).toEqual([
+      {
+        kind: 'function',
+        value: 'theme',
+        nodes: [
+          { kind: 'word', value: 'colors.red.500' },
+          { kind: 'separator', value: '/' },
+          { kind: 'function', value: 'var', nodes: [{ kind: 'word', value: '--opacity' }] },
+        ],
+      },
+    ])
+  })
+
   it('should handle calculations', () => {
     expect(parse('calc((1 + 2) * 3)')).toEqual([
       {
diff --git a/packages/tailwindcss/src/value-parser.ts b/packages/tailwindcss/src/value-parser.ts
index e941a966403b..852ba6ed02e1 100644
--- a/packages/tailwindcss/src/value-parser.ts
+++ b/packages/tailwindcss/src/value-parser.ts
@@ -114,6 +114,7 @@ const SPACE = 0x20
 const LESS_THAN = 0x3c
 const GREATER_THAN = 0x3e
 const EQUALS = 0x3d
+const SLASH = 0x2f
 
 export function parse(input: string) {
   input = input.replaceAll('\r\n', '\n')
@@ -143,6 +144,7 @@ export function parse(input: string) {
       case COLON:
       case COMMA:
       case SPACE:
+      case SLASH:
       case LESS_THAN:
       case GREATER_THAN:
       case EQUALS: {