A spokesman for the prison, north of San Francisco, said nearly 200 inmates rioted in the dining hall.
Four of the men ended up with stab or slash wounds.
Guards stopped the brawl using non-lethal bullets and pepper spray. They found at least 10 weapons made by inmates in the aftermath. No guards were hurt.
There's no word on what sparked the rioting.
San Quentin is California's oldest prison and houses nearly 5,000 men, including those on death row.
The riot came just two days after another riot at California State Prison, Sacramento, left six inmates injured, including two seriously, prison officials said.
The Associated Press contributed to this report.