Greasemonkey，简称GM，中文俗称为“油猴子”，是Mozilla Firefox的一个附加组件。它让用户安装一些脚本使大部分HTML为主的网页于用户端直接改变得更方便易用。随着Greasemonkey脚本常驻于浏览器，每次随着目的网页开启而自动做修改，使得运行脚本的用户印象深刻地享受其固定便利性。

Greasemonkey可替网页加入些新功能（例如在亚马逊书店嵌入商品比价功能）、修正网页错误、组合来自不同网页的数据、或者数繁不及备载的其他功能。写的好的Greasemonkey脚本甚至可让其输出与被修改的页面集成得天衣无缝，像是原本网页里的一部分。

Greasemonkey is a Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to web page content after or before the page is loaded in the browser (also known as augmented browsing).

The changes made to the web pages are executed every time the page is viewed, making them effectively permanent for the user running the script.