Benyan Sabbar al-Obeidi, an organizer of Sunni protests, was shot and killed while he was leaving his house in southern Baghdad, Xinhua News Agency said.
Obeidi just survived another assassination attempt last month, Xinhua said.
A member of the government-backed Awakening Council group was also shot dead in a separate attack Sunday.
The group consists of anti-U.S. Sunni insurgent militant groups, Xinhua said.
Gunmen also killed an engineer while he was leaving work during another attack, Xinhua said.