Dorsey, 29, is expected to be out for the season with a torn left biceps, which he had surgically repaired on Aug. 4. He had 41 tackles and two sacks last season, his first with the 49ers after five with the Chiefs.
Terms of the deal were not disclosed. The fifth pick of the 2008 draft out of LSU signed a two-year deal with San Francisco in March, 2013.
"[Dorsey] is a quick study and a very good football player that has earned this extension," 49ers general manager Trent Baalke said in a statement.
- Chris Mascaro