diff --git a/ChangeLog b/ChangeLog
index 6220122646..45a9b3599f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Aug 27 16:20:01 2010  URABE Shyouhei  <shyouhei@ruby-lang.org>
+
+	* string.c (rb_str_prepend): new method by Sora Harakami
+	 [Feature #3765]
+
 Fri Aug 27 15:24:20 2010  NAKAMURA Usaku  <usa@ruby-lang.org>
 
 	* math.c (math_atan2): you should know that M_PI is not the feature
diff --git a/string.c b/string.c
index d18f47ee46..a0d3b60188 100644
--- a/string.c
+++ b/string.c
@@ -2056,6 +2056,26 @@ rb_str_concat(VALUE str1, VALUE str2)
     }
 }
 
+/*
+ *  call-seq:
+ *     str.prepend(other_str)  -> str
+ *
+ *  Prepend---Prepend the given string to <i>str</i>.
+ *
+ *  a = "world"
+ *  a.prepend("hello ") #=> "hello world"
+ *  a                   #=> "hello world"
+ */
+
+static VALUE
+rb_str_prepend(VALUE str, VALUE str2)
+{
+    StringValue(str2);
+    StringValue(str);
+    rb_str_update(str, 0L, 0L, str2);
+    return str;
+}
+
 st_index_t
 rb_memhash(const void *ptr, long len)
 {
@@ -7525,6 +7545,7 @@ Init_String(void)
     rb_define_method(rb_cString, "reverse!", rb_str_reverse_bang, 0);
     rb_define_method(rb_cString, "concat", rb_str_concat, 1);
     rb_define_method(rb_cString, "<<", rb_str_concat, 1);
+    rb_define_method(rb_cString, "prepend", rb_str_prepend, 1);
     rb_define_method(rb_cString, "crypt", rb_str_crypt, 1);
     rb_define_method(rb_cString, "intern", rb_str_intern, 0);
     rb_define_method(rb_cString, "to_sym", rb_str_intern, 0);
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index 02271cefe6..6fc3167f45 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -1882,4 +1882,20 @@ class TestString < Test::Unit::TestCase
       assert_equal('"\\u3042\\u3044\\u3046"', "\u3042\u3044\u3046".encode(e).inspect)
     end
   end
+
+  def test_prepend
+    assert_equal(S("hello world!"), "world!".prepend("hello "))
+
+    foo = Object.new
+    def foo.to_str
+      "b"
+    end
+    assert_equal(S("ba"), "a".prepend(foo))
+
+    a = S("world")
+    b = S("hello ")
+    a.prepend(b)
+    assert_equal(S("hello world"), a)
+    assert_equal(S("hello "), b)
+  end
 end