Dorsey, who underwent surgery on Aug. 4, was reportedly expected to miss the entire season.
The 49ers on Tuesday announced they have signed Dorsey to a two-year contract extension through the 2016 season. The deal is reportedly worth $6.1 million in new money, with $2.6 million guaranteed.
A first-round pick out of LSU in 2008, Dorsey, 29, recorded 30 tackles and two sacks with the 49ers last season after spending the previous five seasons with the Kansas City Chiefs.
The 49ers open the regular season Sept. 7 against the Dallas Cowboys.
Jim Harbaugh said "a very good chance" that NT Glenn Dorsey (biceps surgery) will return late this season after 3 1/2 months. — Matt Maiocco (@MaioccoCSN) August 22, 2014