机枪或称机关枪、机关铳（Machine gun）指全自动、可快速连续发射的枪械。通常分为轻机枪、中型机枪、通用机枪、重机枪等几种，口径20毫米以上的叫「机炮」（autocannons）。

在一般的法律和口语中，机枪泛指可连发枪械。

机枪为了满足连续射击的稳定需要，通常备有两脚架及可安装在三脚架或固定枪座上，主要发射步枪或更大口径（12.7mm/14.5mm）的子弹，能快速连续射击，以扫射为主要攻击方式，透过绵密火网压制对方火力点或掩护己方进攻。除了攻击有生目标之外，也可以射击其他无装甲防护或薄装甲防护的目标。

A machine gun is a fully automatic mounted or portable firearm, designed to fire bullets in quick succession from an ammunition belt or magazine, typically at a rate of 300 to 1800 rounds per minute. Fully automatic firearms are generally categorized as submachine guns, assault rifles, battle rifles, automatic shotguns, machine guns, or autocannons. Machine guns with multiple rotating barrels are referred to as "rotary machine guns."