One piece spas, also known as unibody spas, are formed as one piece with shapes that provide a variety of seating arrangements within the tub. Each integral seat is often equipped with one or more water jets that allows water to be directed at parts of the body. The water flow may be aerated for additional effect, and some or all of the jets may also automatically move or rotate, providing a massage-like effect.

Although wooden tubs were the most common type of hot tub in the 1970s, one-piece hot tubs now dominate the market as they are less expensive to manufacture, easier to install, and more energy efficient. In America, these kinds of tubs are also referred to as Jacuzzi or Whirlpool tubs, though both are brand names.

Spas usually have between one and four water pumps, with one circulation pump serving the heating and filtration water loop and the other(s) driving the hydrotherapy jets. Sophisticated computer controls are now common and many tubs now are equipped with extensive lighting, sound systems, and even flat panel televisions with DVD players.