The unidentified manager was safe following gunfire that wounded the suspect in Saehan Bank, Buena Park police Cpl. Andy Luong said.
The gunman was in custody, but Luong didn't know the extent of his injuries. TV images showed firefighters tending to a man outside the bank.
"The suspect is down," Luong told reporters. "He's still uncooperative."
The gunman entered the bank shortly after 11 a.m. and ordered everyone out except the manager. It was not known if it was a robbery attempt.
The gunman's motive remains unclear.