diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index 64e9c2950..aa22b2bd0 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -1243,6 +1243,45 @@ mrb_str_lines(mrb_state *mrb, mrb_value self)
   return result;
 }
 
+/*
+ * call-seq:
+ *   +string -> new_string or self
+ *
+ * Returns +self+ if +self+ is not frozen.
+ *
+ * Otherwise returns <tt>self.dup</tt>, which is not frozen.
+ */
+static mrb_value
+mrb_str_uplus(mrb_state *mrb, mrb_value str)
+{
+  if (mrb_frozen_p(mrb_obj_ptr(str))) {
+    return mrb_str_dup(mrb, str);
+  }
+  else {
+    return str;
+  }
+}
+
+/*
+ * call-seq:
+ *   -string -> frozen_string
+ *
+ * Returns a frozen, possibly pre-existing copy of the string.
+ *
+ * The returned \String will be deduplicated as long as it does not have
+ * any instance variables set on it and is not a String subclass.
+ *
+ * String#dedup is an alias for String#-@.
+ */
+static mrb_value
+mrb_str_uminus(mrb_state *mrb, mrb_value str)
+{
+  if (mrb_frozen_p(mrb_obj_ptr(str))) {
+    return str;
+  }
+  return mrb_obj_freeze(mrb, mrb_str_dup(mrb, str));
+}
+
 void
 mrb_mruby_string_ext_gem_init(mrb_state* mrb)
 {
@@ -1278,6 +1317,8 @@ mrb_mruby_string_ext_gem_init(mrb_state* mrb)
   mrb_define_method(mrb, s, "delete_suffix",   mrb_str_del_suffix,      MRB_ARGS_REQ(1));
   mrb_define_method(mrb, s, "casecmp",         mrb_str_casecmp,         MRB_ARGS_REQ(1));
   mrb_define_method(mrb, s, "casecmp?",        mrb_str_casecmp_p,       MRB_ARGS_REQ(1));
+  mrb_define_method(mrb, s, "+@",              mrb_str_uplus,           MRB_ARGS_REQ(1));
+  mrb_define_method(mrb, s, "-@",              mrb_str_uminus,          MRB_ARGS_REQ(1));
 
   mrb_define_method(mrb, s, "__lines",         mrb_str_lines,           MRB_ARGS_NONE());
 
diff --git a/mrbgems/mruby-string-ext/test/string.rb b/mrbgems/mruby-string-ext/test/string.rb
index eb731c503..9de02c59f 100644
--- a/mrbgems/mruby-string-ext/test/string.rb
+++ b/mrbgems/mruby-string-ext/test/string.rb
@@ -1,3 +1,4 @@
+# coding: utf-8
 ##
 # String(Ext) Test
 
@@ -712,3 +713,17 @@ assert('String#delete_suffix') do
   assert_equal "he", "hello".delete_suffix!("llo")
   assert_nil "hello".delete_suffix!("he")
 end
+
+assert('String#+@') do
+  a = +"abc"
+  assert_false(a.frozen?)
+  a = +(a.freeze)
+  assert_false(a.frozen?)
+end
+
+assert('String#-@') do
+  a = -"abc"
+  assert_true(a.frozen?)
+  a = -(a.freeze)
+  assert_true(a.frozen?)
+end