diff --git a/.changeset/loud-cups-tickle.md b/.changeset/loud-cups-tickle.md
new file mode 100644
index 0000000000..122717f5e8
--- /dev/null
+++ b/.changeset/loud-cups-tickle.md
@@ -0,0 +1,16 @@
+---
+'@backstage/ui': minor
+---
+
+**BREAKING**: Data attributes rendered by components are now always lowercase. This affects CSS selectors targeting camelCase data attributes.
+
+**Migration:**
+
+Update any custom CSS selectors that target camelCase data attributes to use lowercase instead:
+
+```diff
+- [data-startCollapsed='true'] { ... }
++ [data-startcollapsed='true'] { ... }
+```
+
+**Affected components:** SearchField
diff --git a/packages/ui/src/components/SearchField/SearchField.module.css b/packages/ui/src/components/SearchField/SearchField.module.css
index da854ee7b1..f4415aa0f8 100644
--- a/packages/ui/src/components/SearchField/SearchField.module.css
+++ b/packages/ui/src/components/SearchField/SearchField.module.css
@@ -39,7 +39,7 @@
       }
     }
 
-    &[data-startCollapsed='true'] {
+    &[data-startcollapsed='true'] {
       transition: flex-basis 0.3s ease-in-out, width 0.3s ease-in-out,
         max-width 0.3s ease-in-out;
       padding: 0;