铅芯笔（英语：Mechanical pencil，propelling pencil），又称自动笔或活动铅笔，是一种固体颜料芯可更换并且可以机械伸缩的铅笔。铅笔芯与外壳不是固定相连的，通常由石墨构成，当它的前端磨损的的时候，可以向外延长。

自动铅笔通常在工程制图和快速工整的书写中可以无需削铅笔就能提供相同宽度的线条。自动铅笔也用于美术绘画中。

自动铅笔在十八世纪第一次使用，十九和二十世纪许多自动铅笔的设计取得了专利。

A mechanical pencil (US English) or a propelling pencil (UK English) is a pencil with a replaceable and mechanically extendable solid pigment core called a "lead" /ˈlɛd/. The lead, often made of graphite, is not bonded to the outer casing, and can be mechanically extended as its point is worn away. Other names include automatic pencil, drafting pencil, technical pencil, click pencil, clutch pencil, leadholder, pen pencil (Indian English), lead pencil (Bangladeshi English), and pacer (or spacer).