diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7614097a3409..d293fd5c130a 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
 
 - Upgrade: migrate arbitrary modifiers with values without percentage sign to bare values `/[0.16]` -> `/16` ([#18184](https://github.com/tailwindlabs/tailwindcss/pull/18184))
 - Upgrade: migrate CSS variable shorthand if fallback value contains function call ([#18184](https://github.com/tailwindlabs/tailwindcss/pull/18184))
+- Upgrade: Migrate negative arbitrary values to negative bare values, e.g.: `mb-[-32rem]` → `-mb-128` ([#18212](https://github.com/tailwindlabs/tailwindcss/pull/18212))
 
 ## [4.1.8] - 2025-05-27
 
diff --git a/packages/@tailwindcss-upgrade/src/codemods/template/migrate-arbitrary-utilities.ts b/packages/@tailwindcss-upgrade/src/codemods/template/migrate-arbitrary-utilities.ts
index 63aa9396d875..3007e18deee3 100644
--- a/packages/@tailwindcss-upgrade/src/codemods/template/migrate-arbitrary-utilities.ts
+++ b/packages/@tailwindcss-upgrade/src/codemods/template/migrate-arbitrary-utilities.ts
@@ -159,7 +159,12 @@ export function migrateArbitraryUtilities(
         candidate.kind === 'arbitrary' ? candidate.value : (candidate.value?.value ?? null)
       if (value === null) return
 
-      let spacingMultiplier = spacing.get(designSystem)?.get(value)
+      let spacingMultiplier = spacing.get(designSystem)?.get(value) ?? null
+      let rootPrefix = ''
+      if (spacingMultiplier !== null && spacingMultiplier < 0) {
+        rootPrefix = '-'
+        spacingMultiplier = Math.abs(spacingMultiplier)
+      }
 
       for (let root of Array.from(designSystem.utilities.keys('functional')).sort(
         // Sort negative roots after positive roots so that we can try
@@ -167,6 +172,8 @@ export function migrateArbitraryUtilities(
         // `-mt-[0px]` can be translated to `mt-[0px]`.
         (a, z) => Number(a[0] === '-') - Number(z[0] === '-'),
       )) {
+        if (rootPrefix) root = `${rootPrefix}${root}`
+
         // Try as bare value
         for (let replacementCandidate of parseCandidate(designSystem, `${root}-${value}`)) {
           yield replacementCandidate
diff --git a/packages/@tailwindcss-upgrade/src/codemods/template/migrate.test.ts b/packages/@tailwindcss-upgrade/src/codemods/template/migrate.test.ts
index 49a40d016834..35de32a09bc2 100644
--- a/packages/@tailwindcss-upgrade/src/codemods/template/migrate.test.ts
+++ b/packages/@tailwindcss-upgrade/src/codemods/template/migrate.test.ts
@@ -69,6 +69,11 @@ describe.each([['default'], ['with-variant'], ['important'], ['prefix']])('%s',
     // the fallback contains functions. The fallback should also be migrated to
     // the newest syntax.
     ['bg-[--my-color,theme(colors.red.500)]', 'bg-(--my-color,var(--color-red-500))'],
+
+    // Both the positive and negative versions of arbitrary utilities should be
+    // converted to the bare value version.
+    ['mb-[32rem]', 'mb-128'],
+    ['mb-[-32rem]', '-mb-128'],
   ])(testName, async (candidate, result) => {
     if (strategy === 'with-variant') {
       candidate = `focus:${candidate}`