According to his attorney, he died at his Malibu, Calif., home Sunday at age 81.
He also played chief surgeon, Dr. John MacIntyre, the title character on "Trapper John, M.D." (1979-1986).
Roberts was also known for his activism, which included participation in the Selma to Montgomery marches in 1965 and pressuring NBC to refrain from hiring whites to portray minority characters.
Roberts, a native of Waycross, Ga., spent much of his adult life "pursuing his dream of equality for all," according to a family statement released by his lawyer, Richard Stone.
"On the set of 'Bonanza,' he protested the use of all white crews and guest stars, finding some support but never enough to satisfy his sense of outrage," the statement said.
Roberts left the popular western after the end of the sixth season in 1965, although "Bonanza" stayed in production for another eight years. The storyline kept open the possibility of his return to the Ponderosa ranch. But Roberts never returned.
In Memoriam: Remembering those we lost in 2009