diff --git a/CHANGELOG.md b/CHANGELOG.md
index 47b21eed24d8..2768ffbc4f44 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure `@plugin` resolves package JavaScript entries instead of browser CSS entries when using `@tailwindcss/vite` ([#19949](https://github.com/tailwindlabs/tailwindcss/pull/19949))
 - Fix relative `@import` and `@plugin` paths resolving from the wrong directory when using `@tailwindcss/vite` ([#19965](https://github.com/tailwindlabs/tailwindcss/pull/19965))
 - Ensure CSS files containing `@variant` are processed by `@tailwindcss/vite` ([#19966](https://github.com/tailwindlabs/tailwindcss/pull/19966))
+- Resolve imports relative to `base` when `result.opts.from` is not provided when using `@tailwindcss/postcss` ([#19980](https://github.com/tailwindlabs/tailwindcss/pull/19980))
 
 ## [4.2.4] - 2026-04-21
 
diff --git a/packages/@tailwindcss-postcss/src/index.test.ts b/packages/@tailwindcss-postcss/src/index.test.ts
index ff12e09ef6af..9d23d0a8ded0 100644
--- a/packages/@tailwindcss-postcss/src/index.test.ts
+++ b/packages/@tailwindcss-postcss/src/index.test.ts
@@ -140,6 +140,16 @@ describe('processing without specifying a base path', () => {
   })
 })
 
+test('fallback to `base` directory when `result.opts.from` is not provided', async () => {
+  let processor = postcss([
+    tailwindcss({ base: `${__dirname}/fixtures/example-project`, optimize: { minify: false } }),
+  ])
+
+  let result = await processor.process(`@import 'tailwindcss'`)
+
+  expect(result.css.length).toBeGreaterThan(0)
+})
+
 describe('plugins', () => {
   test('local CJS plugin', async () => {
     let processor = postcss([
diff --git a/packages/@tailwindcss-postcss/src/index.ts b/packages/@tailwindcss-postcss/src/index.ts
index 2ff4405b66b6..0bc70cb9cf94 100644
--- a/packages/@tailwindcss-postcss/src/index.ts
+++ b/packages/@tailwindcss-postcss/src/index.ts
@@ -115,7 +115,7 @@ function tailwindcss(opts: PluginOptions = {}): AcceptedPlugin {
           }
 
           let context = getContextFromCache(postcss, inputFile, opts)
-          let inputBasePath = path.dirname(path.resolve(inputFile))
+          let inputBasePath = inputFile ? path.dirname(path.resolve(inputFile)) : base
 
           // Whether this is the first build or not, if it is, then we can
           // optimize the build by not creating the compiler until we need it.