A waterbed or water mattress is a bed or mattress filled with water.

Waterbeds primarily consist of two types, hard-sided beds and soft-sided beds.

A hard-sided waterbed consists of a water-containing mattress inside a rectangular frame of wood resting on a plywood deck that sits on a platform. Platforms may incorporate drawer systems for storage. Frames may be decorative, with elaborate headboards or tall posts.

A soft-sided waterbed consists of a water-containing mattress inside of a rectangular frame of sturdy foam, zippered inside of a fabric casing, which sits on a platform. The effect is to look like a conventional bed and is designed to fit into existing bedroom furniture. The platform usually looks like a conventional foundation or box spring, and sits atop a much-stronger-than-normal metal frame.

The bed can be heated, which makes it very comfortable in the winter. Falling asleep is easier, possibly assisted by lowered blood pressure that comes from resting on a warmer environment. The warmth of the water also helps muscles to relax, resulting in improved blood circulation, while alleviating soreness of stiff muscles and joints.

Heating a waterbed is costly. A waterbed consumes between 500 and 2000 kWh/year, depending on the climate, bed size, and other factors. Thus, a waterbed can easily consume more power than a refrigerator. The energy usage can be decreased by about 60% with the use of a foam mattress pad, if the water is not to be heated to body temperature.