diff --git a/include/mruby/string.h b/include/mruby/string.h
index ac6740c5d..329f323b5 100644
--- a/include/mruby/string.h
+++ b/include/mruby/string.h
@@ -92,6 +92,8 @@ struct RStringEmbed {
 # define RSTR_COPY_SINGLE_BYTE_FLAG(dst, src) (void)0
 #endif
 #define RSTR_SET_ASCII_FLAG(s) RSTR_SET_SINGLE_BYTE_FLAG(s)
+#define RSTR_BINARY_P(s) ((s)->flags & MRB_STR_BINARY)
+#define RSTR_SET_BINARY_FLAG(s) ((s)->flags |= MRB_STR_BINARY)
 
 /**
  * Returns a pointer from a Ruby string
@@ -109,7 +111,8 @@ struct RStringEmbed {
 #define MRB_STR_FSHARED   2
 #define MRB_STR_NOFREE    4
 #define MRB_STR_EMBED     8  /* type flags up to here */
-#define MRB_STR_SINGLE_BYTE    16
+#define MRB_STR_BINARY    16
+#define MRB_STR_SINGLE_BYTE    32
 #define MRB_STR_EMBED_LEN_SHIFT 6
 #define MRB_STR_EMBED_LEN_BIT 5
 #define MRB_STR_EMBED_LEN_MASK (((1 << MRB_STR_EMBED_LEN_BIT) - 1) << MRB_STR_EMBED_LEN_SHIFT)
diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index 8ca266b8a..31de0109c 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -1325,6 +1325,13 @@ str_ascii_only_p(mrb_state *mrb, mrb_value str)
   return mrb_true_value();
 }
 
+static mrb_value
+str_b(mrb_state *mrb, mrb_value str)
+{
+  RSTR_SET_BINARY_FLAG(mrb_str_ptr(str));
+  return str;
+}
+
 void
 mrb_mruby_string_ext_gem_init(mrb_state* mrb)
 {
@@ -1364,6 +1371,7 @@ mrb_mruby_string_ext_gem_init(mrb_state* mrb)
   mrb_define_method_id(mrb, s, MRB_OPSYM(minus),          str_uminus,          MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, s, MRB_SYM_Q(valid_encoding), str_valid_enc_p,     MRB_ARGS_NONE());
   mrb_define_method_id(mrb, s, MRB_SYM_Q(ascii_only),     str_ascii_only_p,    MRB_ARGS_NONE());
+  mrb_define_method_id(mrb, s, MRB_SYM(b),                str_b,               MRB_ARGS_NONE());
 
   mrb_define_method_id(mrb, s, MRB_SYM(__lines),          str_lines,           MRB_ARGS_NONE());
   mrb_define_method_id(mrb, s, MRB_SYM(__codepoints),     str_codepoints,      MRB_ARGS_NONE());