diff --git a/.changeset/gold-lions-stick.md b/.changeset/gold-lions-stick.md
new file mode 100644
index 0000000000..da18dd6cf6
--- /dev/null
+++ b/.changeset/gold-lions-stick.md
@@ -0,0 +1,5 @@
+---
+'@backstage/ui': patch
+---
+
+Fixed `useDefinition` hook adding literal "undefined" class name when no className prop was passed.
diff --git a/packages/ui/src/hooks/useDefinition/useDefinition.tsx b/packages/ui/src/hooks/useDefinition/useDefinition.tsx
index 7060ef4a1a..0de2eb8358 100644
--- a/packages/ui/src/hooks/useDefinition/useDefinition.tsx
+++ b/packages/ui/src/hooks/useDefinition/useDefinition.tsx
@@ -103,10 +103,8 @@ export function useDefinition<
     classes[name] = clsx(
       cssKey as string,
       definition.styles[cssKey as keyof typeof definition.styles],
-      {
-        [utilityClasses]: utilityTarget === name,
-        [ownPropsResolved.className]: classNameTarget === name,
-      },
+      utilityTarget === name && utilityClasses,
+      classNameTarget === name && ownPropsResolved.className,
     );
   }