二手烟，亦称非自愿性吸烟，是指在吸取燃点烟草时随着烟雾释放出来的物质，是一种被动吸烟（Passive smoking）方式，也是目前危害最广泛、最严重的室内空气污染。

二手烟包括由吸烟者呼出的烟雾，以及燃烧香烟所产生的烟雾。

有研究指二手烟有焦油、阿摩尼亚、尼古丁、悬浮微粒、超细悬浮微粒、210等超过4000种有害化学物质及数十种致癌物质，吸二手烟的人小便中可被检出。分流烟的毒性比一手烟高，原因包括分流烟是由比主流烟更不完全的燃烧所产生的、而且没有经过滤嘴过滤；因此实际上有不少吸烟者也会厌恶二手烟、甚至有可能因此坚持在通风处吸烟。

Passive smoking is the inhalation of smoke, called second-hand smoke (SHS), or environmental tobacco smoke (ETS), by persons other than the intended "active" smoker. It occurs when tobacco smoke permeates any environment, causing its inhalation by people within that environment. Exposure to second-hand tobacco smoke causes disease, disability, and death. The health risks of second-hand smoke are a matter of scientific consensus. These risks have been a major motivation for smoke-free laws in workplaces and indoor public places, including restaurants, bars and night clubs as well as some open public spaces.