diff --git a/CHANGELOG.md b/CHANGELOG.md
index d590b614d..f6f62b602 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Bring back type exports for the cjs build of `@tailwindcss/postcss`. ([#14256](https://github.com/tailwindlabs/tailwindcss/pull/14256))
+- Correctly merge tuple values when using the plugin API ([#14260](https://github.com/tailwindlabs/tailwindcss/pull/14260))
 
 ## [4.0.0-alpha.20] - 2024-08-23
 
diff --git a/packages/tailwindcss/src/functions.test.ts b/packages/tailwindcss/src/functions.test.ts
index 369785136..91d7fe6b4 100644
--- a/packages/tailwindcss/src/functions.test.ts
+++ b/packages/tailwindcss/src/functions.test.ts
@@ -265,8 +265,8 @@ describe('theme function', () => {
           expect(
             await compileCss(css`
               @theme {
-                --font-size-xs: 0.75rem;
-                --font-size-xs--line-height: 1rem;
+                --font-size-xs: 1337.75rem;
+                --font-size-xs--line-height: 1337rem;
               }
               .text {
                 font-size: theme(fontSize.xs);
@@ -275,13 +275,13 @@ describe('theme function', () => {
             `),
           ).toMatchInlineSnapshot(`
             ":root {
-              --font-size-xs: .75rem;
-              --font-size-xs--line-height: 1rem;
+              --font-size-xs: 1337.75rem;
+              --font-size-xs--line-height: 1337rem;
             }
 
             .text {
-              font-size: .75rem;
-              line-height: 1rem;
+              font-size: 1337.75rem;
+              line-height: 1337rem;
             }"
           `)
         })
diff --git a/packages/tailwindcss/src/theme-fn.ts b/packages/tailwindcss/src/theme-fn.ts
index 5e066bc63..08fd8d6e1 100644
--- a/packages/tailwindcss/src/theme-fn.ts
+++ b/packages/tailwindcss/src/theme-fn.ts
@@ -20,7 +20,7 @@ export function createThemeFn(
 
     let configValue = resolveValue(get(configTheme() ?? {}, keypath) ?? null)
 
-    if (configValue !== null && typeof configValue === 'object') {
+    if (configValue !== null && typeof configValue === 'object' && !Array.isArray(configValue)) {
       return deepMerge({}, [configValue, cssValue], (_, b) => b)
     }