He died Sunday at his Malibu home, wife Eleanor Criswell told the paper.
He played the eldest son of the ranching family headed by Ben Cartwright (Lorne Greene). But he left the top-rated series because he didn't like the development — or lack thereof — of his character.
"They told me the four characters (Greene, himself and Dan Blocker and Michael Landon as his brothers) would be carefully defined and the scripts carefully prepared," he told The Associated Press in 1964. "None of it ever happened."
It bothered him that a man in his 30s had to be so deferential to his widowed father. Roberts fulfilled his six-year contract, then left the series in 1965.
Bonanza, which started in 1959, continued until 1973, making it second to Gunsmoke as the longest-running Western on network television. (Blocker died in 1972, Greene in 1987 and Landon in 1991.)
After appearing for years as guests on other series, Roberts eventually got another TV series: Trapper John, M.D. in 1979. His title role was a much older version of the M*A*S*H character played by Wayne Rogers. It aired until 1986.