diff --git a/mrbgems/mruby-string-ext/mrblib/string.rb b/mrbgems/mruby-string-ext/mrblib/string.rb
index d4f717dea..572418313 100644
--- a/mrbgems/mruby-string-ext/mrblib/string.rb
+++ b/mrbgems/mruby-string-ext/mrblib/string.rb
@@ -36,77 +36,7 @@ class String
     end
   end
 
-  ##
-  # call-seq:
-  #    str.slice!(fixnum)           -> new_str or nil
-  #    str.slice!(fixnum, fixnum)   -> new_str or nil
-  #    str.slice!(range)            -> new_str or nil
-  #    str.slice!(other_str)        -> new_str or nil
-  #
-  # Deletes the specified portion from <i>str</i>, and returns the portion
-  # deleted.
-  #
-  #    string = "this is a string"
-  #    string.slice!(2)        #=> "i"
-  #    string.slice!(3..6)     #=> " is "
-  #    string.slice!("r")      #=> "r"
-  #    string                  #=> "thsa sting"
-  #
-  def slice!(arg1, arg2=nil)
-    raise FrozenError, "can't modify frozen String" if frozen?
-    raise ArgumentError, "wrong number of arguments (expected 1..2)" if arg1.nil? && arg2.nil?
 
-    if !arg1.nil? && !arg2.nil?
-      idx = arg1
-      idx += self.size if arg1 < 0
-      if idx >= 0 && idx <= self.size && arg2 > 0
-        str = self[idx, arg2]
-      else
-        return nil
-      end
-    else
-      validated = false
-      if arg1.kind_of?(Range)
-        beg = arg1.begin
-        ed = arg1.end
-        beg += self.size if beg < 0
-        ed += self.size if ed < 0
-        ed -= 1 if arg1.exclude_end?
-        validated = true
-      elsif arg1.kind_of?(String)
-        validated = true
-      else
-        idx = arg1
-        idx += self.size if arg1 < 0
-        validated = true if idx >=0 && arg1 < self.size
-      end
-      if validated
-        str = self[arg1]
-      else
-        return nil
-      end
-    end
-    unless str.nil? || str == ""
-      if !arg1.nil? && !arg2.nil?
-        idx = arg1 >= 0 ? arg1 : self.size+arg1
-        str2 = self[0...idx] + self[idx+arg2..-1].to_s
-      else
-        if arg1.kind_of?(Range)
-          idx = beg >= 0 ? beg : self.size+beg
-          idx2 = ed>= 0 ? ed : self.size+ed
-          str2 = self[0...idx] + self[idx2+1..-1].to_s
-        elsif arg1.kind_of?(String)
-          idx = self.index(arg1)
-          str2 = self[0...idx] + self[idx+arg1.size..-1] unless idx.nil?
-        else
-          idx = arg1 >= 0 ? arg1 : self.size+arg1
-          str2 = self[0...idx] + self[idx+1..-1].to_s
-        end
-      end
-      self.replace(str2) unless str2.nil?
-    end
-    str
-  end
 
   ##
   #  call-seq:
diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index 78e3f39d3..f96b55ab9 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -1843,6 +1843,147 @@ str_center_core(mrb_state *mrb, mrb_value self)
   return mrb_str_cat_str(mrb, result, right_padding);
 }
 
+#ifdef MRB_UTF8_STRING
+/*
+ * Given a character index, find the byte offset in a UTF-8 string.
+ * Returns -1 if the character index is out of bounds.
+ */
+static mrb_int
+str_char_to_byte_offset(mrb_value str, mrb_int char_index)
+{
+  struct RString *s = mrb_str_ptr(str);
+  const char *p = RSTR_PTR(s);
+  mrb_int byte_len = RSTR_LEN(s);
+
+  if (RSTR_SINGLE_BYTE_P(s) || RSTR_BINARY_P(s)) {
+    return char_index;
+  }
+
+  if (char_index < 0) return -1;
+
+  mrb_int byte_offset = 0;
+  mrb_int current_char_index = 0;
+  while (byte_offset < byte_len && current_char_index < char_index) {
+    mrb_int char_len = mrb_utf8len(p + byte_offset, p + byte_len - byte_offset);
+    if (char_len == 0) break;
+    byte_offset += char_len;
+    current_char_index++;
+  }
+
+  if (current_char_index < char_index) return -1;
+  return byte_offset;
+}
+
+/*
+ * Given a starting character index and a character length, find the byte length.
+ */
+static mrb_int
+str_chars_to_byte_len(mrb_value str, mrb_int char_start, mrb_int char_len)
+{
+  struct RString *s = mrb_str_ptr(str);
+  const char *p = RSTR_PTR(s);
+  mrb_int str_byte_len = RSTR_LEN(s);
+
+  if (RSTR_SINGLE_BYTE_P(s) || RSTR_BINARY_P(s)) {
+    return char_len;
+  }
+
+  mrb_int start_byte_offset = str_char_to_byte_offset(str, char_start);
+  if (start_byte_offset == -1) return 0;
+
+  mrb_int byte_offset = start_byte_offset;
+  mrb_int current_char_len = 0;
+  while (byte_offset < str_byte_len && current_char_len < char_len) {
+    mrb_int cl = mrb_utf8len(p + byte_offset, p + str_byte_len - byte_offset);
+    if (cl == 0) break;
+    byte_offset += cl;
+    current_char_len++;
+  }
+
+  return byte_offset - start_byte_offset;
+}
+#endif
+
+static mrb_value
+mrb_str_slice_bang(mrb_state *mrb, mrb_value self)
+{
+  mrb_check_frozen(mrb, mrb_obj_ptr(self));
+
+  mrb_value arg1, arg2;
+  mrb_int argc = mrb_get_args(mrb, "o|o", &arg1, &arg2);
+
+  struct RString *str = mrb_str_ptr(self);
+  mrb_int str_len;
+  const char *ptr = RSTRING_PTR(self);
+
+#ifdef MRB_UTF8_STRING
+  str_len = str_char_count(self);
+#else
+  str_len = RSTRING_LEN(self);
+#endif
+
+  mrb_int beg, len;
+
+  if (argc == 1) {
+    if (mrb_string_p(arg1)) {
+      mrb_int pos = mrb_str_index(mrb, self, RSTRING_PTR(arg1), RSTRING_LEN(arg1), 0);
+      if (pos == -1) return mrb_nil_value();
+#ifdef MRB_UTF8_STRING
+      beg = str_char_count(mrb_str_substr(mrb, self, 0, pos));
+      len = str_char_count(arg1);
+#else
+      beg = pos;
+      len = RSTRING_LEN(arg1);
+#endif
+    }
+    else if (mrb_range_p(arg1)) {
+      if (mrb_range_beg_len(mrb, arg1, &beg, &len, str_len, TRUE) != MRB_RANGE_OK) {
+        return mrb_nil_value();
+      }
+    }
+    else {
+      beg = mrb_as_int(mrb, arg1);
+      if (beg < 0) beg += str_len;
+      if (beg < 0 || beg >= str_len) return mrb_nil_value();
+      len = 1;
+    }
+  }
+  else { // argc == 2
+    beg = mrb_as_int(mrb, arg1);
+    len = mrb_as_int(mrb, arg2);
+    if (beg < 0) beg += str_len;
+    if (len < 0) return mrb_nil_value();
+    if (beg < 0 || beg > str_len) return mrb_nil_value();
+  }
+
+  if (beg > str_len) return mrb_nil_value();
+  if (beg + len > str_len) {
+    len = str_len - beg;
+  }
+  if (len < 0) len = 0;
+
+#ifdef MRB_UTF8_STRING
+  mrb_int byte_beg = str_char_to_byte_offset(self, beg);
+  mrb_int byte_len = str_chars_to_byte_len(self, beg, len);
+#else
+  mrb_int byte_beg = beg;
+  mrb_int byte_len = len;
+#endif
+
+  if (byte_beg < 0 || byte_beg > RSTRING_LEN(self) || byte_beg + byte_len > RSTRING_LEN(self)) {
+    return mrb_nil_value();
+  }
+
+  mrb_value result = mrb_str_new(mrb, RSTRING_PTR(self) + byte_beg, byte_len);
+
+  mrb_str_modify(mrb, str);
+  ptr = RSTRING_PTR(self);
+  memmove((char*)ptr + byte_beg, ptr + byte_beg + byte_len, RSTRING_LEN(self) - byte_beg - byte_len);
+  RSTR_SET_LEN(str, RSTRING_LEN(self) - byte_len);
+
+  return result;
+}
+
 void
 mrb_mruby_string_ext_gem_init(mrb_state* mrb)
 {
@@ -1850,6 +1991,7 @@ mrb_mruby_string_ext_gem_init(mrb_state* mrb)
 
   mrb_define_method_id(mrb, s, MRB_SYM(dump),             mrb_str_dump,        MRB_ARGS_NONE());
   mrb_define_method_id(mrb, s, MRB_SYM_B(swapcase),       str_swapcase_bang,   MRB_ARGS_NONE());
+  mrb_define_method_id(mrb, s, MRB_SYM_B(slice), mrb_str_slice_bang, MRB_ARGS_ARG(1, 1));
   mrb_define_method_id(mrb, s, MRB_SYM(swapcase),         str_swapcase,        MRB_ARGS_NONE());
   mrb_define_method_id(mrb, s, MRB_SYM(concat),           str_concat_m,        MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, s, MRB_OPSYM(lshift),         str_concat_m,        MRB_ARGS_REQ(1));