A bath, bathtub, or tub (informal) is a large container for holding water in which a person may bathe (take a bath). Most modern bathtubs are made of acrylic or fiberglass, but alternatives are available in enamel over steel or cast iron, and occasionally waterproof finished wood. A bathtub is usually placed in a bathroom either as a stand-alone fixture or in conjunction with a shower.

Modern bathtubs have overflow and waste drains and may have taps mounted on them. They may be built-in or freestanding or sometimes sunken. Until recently, most bathtubs were roughly rectangular in shape but with the advent of acrylic thermoformed baths, more shapes are becoming available. Bathtubs are commonly white in colour although many other colours can be found. The process for enameling cast iron bathtubs was invented by the Scottish-born American David Dunbar Buick.

Two main styles of the bathtub are common:

• Western-style bathtubs in which the bather lies down. These baths are typically shallow and long.
• Eastern style bathtubs in which the bather stands up. These are known as ofuro in Japan and are typically short and deep.