diff --git a/iseq.c b/iseq.c
index a60228c8e585eb..19be0b3e1643dc 100644
--- a/iseq.c
+++ b/iseq.c
@@ -114,7 +114,9 @@ remove_from_constant_cache(ID id, IC ic)
         st_table *ics = (st_table *)lookup_result;
         st_delete(ics, &ic_data, NULL);
 
-        if (ics->num_entries == 0) {
+        if (ics->num_entries == 0 &&
+                // See comment in vm_track_constant_cache on why we need this check
+                id != vm->inserting_constant_cache_id) {
             rb_id_table_delete(vm->constant_cache, id);
             st_free_table(ics);
         }
diff --git a/vm_core.h b/vm_core.h
index bce5f35763383c..d05a9e73439656 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -773,6 +773,7 @@ typedef struct rb_vm_struct {
     // and Qtrue as values. It is used when inline constant caches need to be
     // invalidated or ISEQs are being freed.
     struct rb_id_table *constant_cache;
+    ID inserting_constant_cache_id;
 
 #ifndef VM_GLOBAL_CC_CACHE_TABLE_SIZE
 #define VM_GLOBAL_CC_CACHE_TABLE_SIZE 1023
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index bd616b2422ca56..f4752a7d97f9a8 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -6279,7 +6279,8 @@ rb_vm_opt_newarray_pack(rb_execution_context_t *ec, rb_num_t num, const VALUE *p
 static void
 vm_track_constant_cache(ID id, void *ic)
 {
-    struct rb_id_table *const_cache = GET_VM()->constant_cache;
+    rb_vm_t *vm = GET_VM();
+    struct rb_id_table *const_cache = vm->constant_cache;
     VALUE lookup_result;
     st_table *ics;
 
@@ -6291,7 +6292,23 @@ vm_track_constant_cache(ID id, void *ic)
         rb_id_table_insert(const_cache, id, (VALUE)ics);
     }
 
+    /* The call below to st_insert could allocate which could trigger a GC.
+     * If it triggers a GC, it may free an iseq that also holds a cache to this
+     * constant. If that iseq is the last iseq with a cache to this constant, then
+     * it will free this ST table, which would cause an use-after-free during this
+     * st_insert.
+     *
+     * So to fix this issue, we store the ID that is currently being inserted
+     * and, in remove_from_constant_cache, we don't free the ST table for ID
+     * equal to this one.
+     *
+     * See [Bug #20921].
+     */
+    vm->inserting_constant_cache_id = id;
+
     st_insert(ics, (st_data_t) ic, (st_data_t) Qtrue);
+
+    vm->inserting_constant_cache_id = (ID)0;
 }
 
 static void