地下室，又名地库，是建筑物、大厦等的低于地面的一层或多层。楼层邮寄地址英文书写为B1、B2等。

地下室在条件上是空气比较不流通、阳光不能照射，是次等的生活空间。所以地下室多数用作货仓、酒库、机房、停车场之类。在寸金尺土的都市，地下室也可能利用作出租房、百货公司、酒吧、商场等。而有些国家会规定战时地下室需做为防空及避难之用。在龙卷风经常发生的美国中部平原地区，住宅等建物会设有地窖供龙卷风来袭时避难用。

在地下城、地下街、地下铁发达的地区，地下室有信道互联，其用途会更加多样化。

但在经常有水灾的地方，地下室有被水淹没的危机，因此在进入地下室的入口需装有防水闸门。

A basement or cellar is one or more floors of a building that are either completely or partially below the ground floor. Basements are generally used as a utility space for a building where such items as the boiler, water heater, breaker panel or fuse box, car park, and air-conditioning system are located; so also are amenities such as the electrical distribution system, and cable television distribution point. However, in cities with high property prices such as London, basements are often fitted out to a high standard and used as living space.