A bathtub, also known simply as a bath or tub, is a container for holding water in which a person or animal may bathe. Most modern bathtubs are made of thermoformed acrylic, porcelain-enameled steel or cast iron, or fiberglass-reinforced polyester. A bathtub is 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 are usually built-in, but may be free-standing or sometimes sunken. Until acrylic thermoforming technology permitted other shapes, virtually all bathtubs used to be roughly rectangular. Bathtubs are commonly white in color, although many other colors can be found.

Two main styles 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 sits up. These are known as furo in Japan and are typically short and deep.