diff a/src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.hpp b/src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.hpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.hpp
@@ -299,11 +299,28 @@
 
   inline static size_t required_regions(size_t bytes) {
     return (bytes + ShenandoahHeapRegion::region_size_bytes() - 1) >> ShenandoahHeapRegion::region_size_bytes_shift();
   }
 
-  inline static bool requires_humongous(size_t words) {
+  // Whether an allocation of the given word size must occupy a contiguous run of
+  // humongous regions rather than fitting in a single region.
+  //
+  // may_expand_for_hash must be set only when 'words' is the *base* size of a
+  // freshly allocated, not-yet-hashed object that could later grow by one word
+  // when an identity hash-code is injected as it is copied during GC (see
+  // oopDesc::copy_size). In that case we account for that potential growth when
+  // deciding whether the object needs the humongous path, so an object whose
+  // expanded form would overflow a region is routed to humongous from the start.
+  // It must be false for every size that is already final:
+  //   - the live size of an existing object (obj->size()), which already includes
+  //     the hash word once the object has been expanded by a prior GC copy;
+  //   - an evacuation/promotion copy size (copy_size()), which is already expanded;
+  //   - a raw LAB buffer (TLAB/GCLAB/PLAB), which never grows.
+  inline static bool requires_humongous(size_t words, bool may_expand_for_hash) {
+    if (UseCompactObjectHeaders && may_expand_for_hash) {
+      words = oopDesc::hash_expanded_size(words);
+    }
     return words > ShenandoahHeapRegion::RegionSizeWords;
   }
 
   inline static size_t region_count() {
     return ShenandoahHeapRegion::RegionCount;