A hot tub is a large tub full of water used for hydrotherapy, relaxation or pleasure. Some have powerful jets for massage purposes. Hot tubs are sometimes also known as "spas" or by the trade name Jacuzzi.

In contrast to a typical bathtub, a hot tub is designed to be used by more than one person at a time, with many models accommodating four or more people. Hot tubs are usually located outdoors, although they can be installed indoors. Another difference between baths and hot tubs is that soaps and shampoos are not used in wet-jetted hot tubs (although they can be used in air-jetted hot tubs). Home hot tubs are often closer in construction to standard bathtubs, while the construction of a public hot tub often has more in common with a swimming pool, of which it can be considered a type.