diff --git a/src/stringio.c b/src/stringio.c
index 5560e00..353aacd 100644
--- a/src/stringio.c
+++ b/src/stringio.c
@@ -468,6 +468,36 @@ stringio_getc(mrb_state *mrb, mrb_value self)
   return ret;
 }
 
+static mrb_value
+stringio_ungetc(mrb_state *mrb, mrb_value self)
+{
+  struct StringIO *ptr = StringIO(self);
+  mrb_value string = stringio_iv_get("@string");
+  mrb_int in_len;
+  const char* in_data;
+  mrb_get_args(mrb, "s", &in_data, &in_len);
+
+  mrb_int flags = mrb_fixnum(stringio_iv_get("@flags"));
+  if ((flags & FMODE_READABLE) == 0) {
+    mrb_raise(mrb, E_IOERROR, "not opened for reading");
+  }
+
+  if (in_len == 0) {
+    return mrb_nil_value();
+  }
+
+  if (in_len != 1) {
+    mrb_raisef(mrb, E_IOERROR, "mruby-ext: ungetc is only supported for single byte string. len: %S", mrb_fixnum_value(in_len));
+  }
+  if (ptr->pos <= 0) {
+    mrb_raise(mrb, E_IOERROR, "mruby-ext: ungetc not supported in beginning of stream");
+  }
+  ptr->pos -= 1;
+
+  mrb_assert(RSTRING_PTR(string)[ptr->pos] == *in_data);
+  return mrb_nil_value();
+}
+
 static mrb_value
 stringio_gets(mrb_state *mrb, mrb_value self)
 {
@@ -653,6 +683,7 @@ mrb_mruby_stringio_gem_init(mrb_state* mrb)
   mrb_define_alias(mrb, stringio, "syswrite", "write");
   mrb_define_method(mrb, stringio, "getc", stringio_getc, MRB_ARGS_ANY());
   mrb_define_method(mrb, stringio, "gets", stringio_gets, MRB_ARGS_ANY());
+  mrb_define_method(mrb, stringio, "ungetc", stringio_ungetc, MRB_ARGS_REQ(1));
   mrb_define_method(mrb, stringio, "seek", stringio_seek, MRB_ARGS_ANY());
   mrb_define_method(mrb, stringio, "size", stringio_size, MRB_ARGS_NONE());
   mrb_define_alias(mrb, stringio, "length", "size");
diff --git a/test/stringio.rb b/test/stringio.rb
index 26c1497..7227289 100644
--- a/test/stringio.rb
+++ b/test/stringio.rb
@@ -206,6 +206,19 @@ def o.to_s; "baz"; end
   assert_equal nil, strio.getc
 end
 
+assert 'StringIO#ungetc' do
+  strio = StringIO.new("abc")
+  strio.ungetc ''
+  assert_equal 'abc', strio.string
+  assert_raise(IOError) { strio.ungetc 'a' }
+  assert_equal 'a', strio.getc
+  assert_nil strio.ungetc 'a'
+  assert_equal 'a', strio.getc
+
+  strio = StringIO.new("abc", 'w')
+  assert_raise(IOError) { strio.ungetc 'a' }
+end
+
 assert 'StringIO#gets' do
   io = StringIO.new("this>is>an>example")
   assert_equal "this>", io.gets(">")