The Riley & Scott Mark III (Mk III) was a sports prototype auto racing car developed by Bob Riley, Bill Riley, and Mark Scott of Riley & Scott Cars Inc.. Initially designed in 1993, the car was created for the World Sports Car (WSC) category which was to debut in the North American IMSA GT Championship during their 1994 season. It was not until 1995 that the first Mk III was completed, but the construction of further cars allowed a variety of teams to campaign in several North American and European racing series, including competing at the 24 Hours of Le Mans.

In 1999, Riley & Scott evolved the Mk III's designs in order to adapt to the newer Le Mans Prototype (LMP) regulations which were now used in several series. An all-new third design officially known as the Mark III Series C debuted in 2001 as the final variant developed by the company before they moved on to other programs. Several private teams also made their own modifications to their Mk IIIs in attempts to improve the car's performance to suit their own needs.

The original Mk IIIs were used in competition until the end of 2002, in the process accumulating 47 overall race victories in both North American and Europe, as well as championship titles in the IMSA GT Championship, United States Road Racing Championship, Rolex Sports Car Series, and American Le Mans Series. Mk III Cs continued to compete until 2005, although they were never able to achieve victories like their earlier predecessor.

In 1993, Bill Riley began initial work on a design for a sports prototype to meet the International Motor Sports Association's (IMSA) World Sports Car regulations. The new regulations, announced during the 1993 and intended to be debut in the 1994 season, sought low-cost open-cockpit prototypes to replace the expensive closed-cockpit GTP models used in the IMSA GT Championship. Riley & Scott, already experienced constructors in the Trans-Am Series, were looking to enter the sports prototype category of motorsports.

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