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Category: Image - :Hot tub.jpg|thumb|right|Hot tub at Big White Ski Resort

Category: Image - :Wood-fired Hot Tub.jpg|thumb|Wood-fired Hot Tub at the Adirondack Canoe Classic, Saranac Lake, NY

A hot tub is a large home-made or manufactured tub or small pool full of heated water and used for soaking, relaxation, massage, or hydrotherapy. In most cases, they have jets for massage purposes. Hot tubs are usually located outdoors, and are often sheltered for protection from the elements, as well as for privacy.

There are two different styles of hot tubs:

* Simple wooden-staved soaking tubs

* One piece plastic tubs (usually referred to as spas)

Hot tubs are usually heated using an electric or natural gas heater, though there are also submersible wood-fired heaters, as well as solar hot water systems. Hot tubs are also found at natural hot springs; in this case, the water may be dangerously hot and must be combined with cool water for a safe soaking temperature.

Hot tubs began to be popularized in the United States in California when instead of driving 2 hours on dirt roads to Caliente Hot Springs behind Santa Barbara, a wine vat that had just been sitting around was filled up with hot water (according to the U.S. News & Report).

Water sanitization is very important in hot tubs, as many organisms thrive in a warm, wet environment. Maintaining the hot tub water chemistry is also necessary for proper sanitization and to prevent damage to the hot tub.

Of the 130-odd manufacturers in North America in 2005, the top twenty each produced more than 7,000 spas annually. China is quickly becoming the largest supplier of Hot tubs.