Police swarmed the Costco after shots were reported at the huge store about 50 miles (80.5 kilometers) from downtown Los Angeles.
Witnesses told KCAL-TV that a man with a Mohawk haircut was arguing with someone near a freezer section when he pulled a gun and fired at least six shots.
The man involved in the argument was killed and two other people were wounded, Lt. Jeff Edwards said. There was no immediate word on their conditions, AP reported.
The suspected gunman was apprehended, claimed to be injured and was taken to the hospital, Edwards said. His name and condition were not immediately released.
Shoppers and employees described terror and chaos as the shots rang out.
Christina Colis told the Riverside Press-Enterprise that she was in the produce area when she heard six to seven shots.
Colis told the paper that she and other shoppers hid in a refrigerated produce room. She said her mother saw people injured on the floor.
Will Lungo, 45, of Corona, said he and his wife were near the produce and alcohol sections when he heard gunshots.
"I thought maybe someone dropped a bottle of wine, but then I kept hearing shots," Lungo told the Press-Enterprise. "An employee came in and helped us out through the emergency exit."
Witnesses told KCAL-TV that shoppers and employees rushed to the exits. At one point, the station reported more than 100 people were outside the store.