diff --git a/include/ruby/encoding.h b/include/ruby/encoding.h
index 07e6c9a671a705..126f2c5c3eb7a9 100644
--- a/include/ruby/encoding.h
+++ b/include/ruby/encoding.h
@@ -235,7 +235,6 @@ rb_enc_asciicompat_inline(rb_encoding *enc)
 }
 #define rb_enc_asciicompat(enc) rb_enc_asciicompat_inline(enc)
 
-int rb_enc_casefold(char *to, const char *p, const char *e, rb_encoding *enc);
 CONSTFUNC(int rb_enc_toupper(int c, rb_encoding *enc));
 CONSTFUNC(int rb_enc_tolower(int c, rb_encoding *enc));
 ID rb_intern3(const char*, long, rb_encoding*);