diff --git a/include/ruby/internal/core/rstring.h b/include/ruby/internal/core/rstring.h
index 7b4953ab8c..e394ab7dca 100644
--- a/include/ruby/internal/core/rstring.h
+++ b/include/ruby/internal/core/rstring.h
@@ -556,9 +556,6 @@ RSTRING_LENINT(VALUE str)
     return rb_long2int(RSTRING_LEN(str));
 }
 
-bool
-rb_str_shared_root_p(VALUE str);
-
 /**
  * Convenient macro to obtain the contents and length at once.
  *
diff --git a/string.c b/string.c
index 68baa91cc0..e06e7745a6 100644
--- a/string.c
+++ b/string.c
@@ -233,12 +233,6 @@ rb_str_embed_size(long capa)
     return offsetof(struct RString, as.embed.ary) + capa;
 }
 
-bool
-rb_str_shared_root_p(VALUE str)
-{
-    return FL_TEST_RAW(str, STR_SHARED_ROOT);
-}
-
 size_t
 rb_str_size_as_embedded(VALUE str)
 {