The International Motorsports Association Grand Touring Championship, or IMSA GT, was a racing series established in 1971 to bring 24 Hours of Le Mans caliber cars to the United States other than in endurance races like the Twelve Hours of Sebring or Six Hours of Watkins Glen. Arriving during the decline of performance sedans, IMSA GT is perhaps responsible for the slow decline of the SCCA from 1973-1989. The series lasted until 1998, being reorganized into the American Le Mans Series in 1999. In 1998, IMSA lost the rights to the 24 Hours of Daytona and Watkins Glen to the newly-revived United States Road Racing Championship. With the merger of Grand Am (which had been created out of the USRRC in 2000) and the ALMS next year, IMSA reacquired those events for its new Tudor Championship
Despite being sanctioned by IMSA, the series only left North America once, with one event being held at Autódromo Hermanos Rodríguez.
- GTP- based on the FIA's group C prototypes. The class was created in 1982 and lasted until 1993, after which they were replaced with "World Sports Cars". These new prototypes would go on to become Le Mans Prototypes, and were carried over into the American Le Mans Series, which replaced IMSA GT in 1999.
- GTO cars were Grand Touring type cars with an engine of 2.5 L displacement or more. The letter "O" colloquially meaning "Over 2.5L". The GTO group was dominated by Chevrolet Corvettes, then by Shelby Mustangs and then various factory teams consisting of Mercury Cougars, Datsun 280Z's, Toyota Celicas and finally the Nissan 300ZX. Especially dominant in GTO was the Audi Quattro, so successful that it was quickly banned from both IMSA and SCCA competition.
- GTU cars were Grand Touring type cars with engines of 2.5 L displacement or less, the letter "U" colloquially meaning "Under 2.5L". The GTU group was dominated by the Porsche 914 and SA22 Mazda RX-7 (1978–1985) through the end of the 1980s. Especially dominant in GTU was the Merkur XR4Ti, particularly those driven by Willy Ribbs.
- GTX cars were based on the FIA Group 5 cars and bore little resemblance to their production counterparts.
- TO were touring-type cars such as the Chevrolet Camaro with an engine of 2.5 L or more displacement. This group was essentially the old Trans Am Series over two liter class.
- TU were touring-type cars with an engine of 2.5 L or less displacement such as the Opel Kadett. This was essentially the old Trans Am under two liter class.
- AAGT (All-American GT)- a category created in 1975 to counteract Porsche's dominance in GTO.
In recent years the Monterey Historic Automobile Races at Mazda Raceway Laguna Seca have featured IMSA race groups on an annual basis. The initial race group featured a mixture of GTO, GTU, GTX, and AAGT cars. An IMSA GTP group was later added. The IMSA GTOs were later put into a separate race group to prevent them from dominating. It should be noted, however, that both the GTP and GTO groups feature Mazdas. This was likely a major contributor to the addition of the groups, as Mazda is the racetrack's corporate sponsor.
Racing video game Forza Motorsport 5 features the IMSA GTO version of the Audi 90 Quattro, the first IMSA GT vehicle in franchise history.