The Lancia Delta S4 is a Group B rally car manufactured by the Italian car company Lancia. The Delta S4 competed in the World Rally Championship in 1985 and 1986, until Group B class was disbanded and the cars were eventually banned from competition completely by European sanctioning body FIA. The car replaced, and was an evolution of the 037. The S4 took full advantage of the Group B regulations, and featured a midship-mounted engine and all-wheel drive for superior traction on loose surfaces.

Group B was a set of regulations for grand touring (GT) vehicles used in sports car racing and rallying introduced in 1982 by the Fédération Internationale de l'Automobile (FIA). Although permitted to enter a GT class of the World Sportscar Championship alongside the more popular racing prototypes of Group C, Group B are commonly associated with the international rallying scene during 1982 to 1986 in popular culture, when they were the highest class used across rallying, including the World Rally Championship, regional and national championships.