diff --git a/cont.c b/cont.c
index d167685f13..c49256c977 100644
--- a/cont.c
+++ b/cont.c
@@ -2005,10 +2005,9 @@ fiber_alloc(VALUE klass)
 }
 
 static rb_serial_t
-next_fiber_serial(void)
+next_fiber_serial(rb_ractor_t *cr)
 {
-    static rbimpl_atomic_uint64_t fiber_serial = 1;
-    return (rb_serial_t)ATOMIC_U64_FETCH_ADD(fiber_serial, 1);
+    return cr->next_fiber_serial++;
 }
 
 static rb_fiber_t*
@@ -2027,7 +2026,7 @@ fiber_t_alloc(VALUE fiber_value, unsigned int blocking)
     fiber->cont.type = FIBER_CONTEXT;
     fiber->blocking = blocking;
     fiber->killed = 0;
-    fiber->serial = next_fiber_serial();
+    fiber->serial = next_fiber_serial(th->ractor);
     cont_init(&fiber->cont, th);
 
     fiber->cont.saved_ec.fiber_ptr = fiber;
@@ -2580,7 +2579,7 @@ rb_threadptr_root_fiber_setup(rb_thread_t *th)
     fiber->cont.saved_ec.thread_ptr = th;
     fiber->blocking = 1;
     fiber->killed = 0;
-    fiber->serial = next_fiber_serial();
+    fiber->serial = next_fiber_serial(th->ractor);
     fiber_status_set(fiber, FIBER_RESUMED); /* skip CREATED */
     th->ec = &fiber->cont.saved_ec;
     cont_init_jit_cont(&fiber->cont);
diff --git a/ractor.c b/ractor.c
index 48fbf7cfb9..3fc507128c 100644
--- a/ractor.c
+++ b/ractor.c
@@ -418,6 +418,7 @@ ractor_alloc(VALUE klass)
     VALUE rv = TypedData_Make_Struct(klass, rb_ractor_t, &ractor_data_type, r);
     FL_SET_RAW(rv, RUBY_FL_SHAREABLE);
     r->pub.self = rv;
+    r->next_fiber_serial = 1;
     VM_ASSERT(ractor_status_p(r, ractor_created));
     return rv;
 }
@@ -435,6 +436,7 @@ rb_ractor_main_alloc(void)
     r->name = Qnil;
     r->pub.self = Qnil;
     r->newobj_cache = rb_gc_ractor_cache_alloc(r);
+    r->next_fiber_serial = 1;
     ruby_single_main_ractor = r;
 
     return r;
diff --git a/ractor_core.h b/ractor_core.h
index 130ccb1118..81374c0769 100644
--- a/ractor_core.h
+++ b/ractor_core.h
@@ -88,6 +88,8 @@ struct rb_ractor_struct {
 
     // ractor local data
 
+    rb_serial_t next_fiber_serial;
+
     st_table *local_storage;
     struct rb_id_table *idkey_local_storage;
     VALUE local_storage_store_lock;
diff --git a/ruby_atomic.h b/ruby_atomic.h
index b60c56cfd8..cbcfe682ce 100644
--- a/ruby_atomic.h
+++ b/ruby_atomic.h
@@ -70,27 +70,4 @@ rbimpl_atomic_u64_set_relaxed(volatile rbimpl_atomic_uint64_t *address, uint64_t
 }
 #define ATOMIC_U64_SET_RELAXED(var, val) rbimpl_atomic_u64_set_relaxed(&(var), val)
 
-static inline uint64_t
-rbimpl_atomic_u64_fetch_add(volatile rbimpl_atomic_uint64_t *ptr, uint64_t val)
-{
-#if defined(HAVE_GCC_ATOMIC_BUILTINS_64)
-    return __atomic_fetch_add(ptr, val, __ATOMIC_SEQ_CST);
-#elif defined(_WIN32)
-    return InterlockedExchangeAdd64((volatile LONG64 *)ptr, val);
-#elif defined(__sun) && defined(HAVE_ATOMIC_H) && (defined(_LP64) || defined(_I32LPx))
-    return atomic_add_64_nv(ptr, val) - val;
-#elif defined(HAVE_STDATOMIC_H)
-    return atomic_fetch_add_explicit((_Atomic uint64_t *)ptr, val, memory_order_seq_cst);
-#else
-    // Fallback using mutex for platforms without 64-bit atomics
-    static rb_nativethread_mutex_t lock = RB_NATIVETHREAD_LOCK_INIT;
-    rb_native_mutex_lock(&lock);
-    uint64_t old = *ptr;
-    *ptr = old + val;
-    rb_native_mutex_unlock(&lock);
-    return old;
-#endif
-}
-#define ATOMIC_U64_FETCH_ADD(var, val) rbimpl_atomic_u64_fetch_add(&(var), val)
-
 #endif