Ricky Bell, an outstanding running back at Southern California who played six years in the National Football League, died yesterday at age 29 in a hospital in Inglewood, Calif., not far from his Los Angeles home. Doctors attributed his death to cardiac arrest caused by illnesses from which he had been suffering for two years.

Bell, who was the first pick in the N.F.L.'s 1977 draft, suffered from dermatomyositis and polymyositis, diseases that together affect the skin, the striated muscles and various connective tissues of the body. Their cause is unknown.

''Dermatomyositis is a chronic, inflammatory muscle disease felt to be due to abnormalities of the human immune system,'' said Dr. Allan Metzger of Beverly Hills, who was Bell's physician. ''Less than five percent of patients with this disease have associated heart disease of this severity.'' Led Nation in Rushing

As a junior in college, Bell led the nation in rushing, with 1,875 yards. A year later, he finished second to Tony Dorsett of Pitt in the balloting for the Heisman Trophy. After his senior year, he was drafted by the Tampa Bay Buccaneers, whose head coach, John McKay, had resigned as coach at Southern Cal after Bell's junior season.

''Ricky Bell was one of the finest football players I've ever had the pleasure of coaching,'' McKay said yesterday in a statement issued by the Buccaneers. ''He was an even finer man. This is a great tragedy. My heart goes out to his family.''

When Bell played at Southern Cal, McKay compared him to another great U.S.C. runner, O. J. Simpson.

''Next to O. J.,'' McKay once said, ''Ricky Bell has the best speed I've ever coached at tailback. And at 6-2 and 215 pounds, he's the biggest tailback I've ever had. He has tremendous power of acceleration. There's no limit to his success, provided he continues to get good blocking. But, with his size and speed, he needs less blocking than other people.''

Set Records for Tampa Bay

In five seasons with the Buccaneers, Bell set several club records, and he still holds the Tampa Bay career rushing record, 3,057 yards. But his playing time dwindled in 1981, partly because of a series of minor injuries that were later thought to mark the beginnings of his illness. He asked to be traded, and in March 1982 McKay sent him to the San Diego Chargers. But, suffering from weight loss, aching muscles and severe skin problems, he retired before the 1983 season.

He is survived by his wife, Natalia; a 3-year-old daughter, Noell, and his mother, Ruth.