The attack was reported around 11 a.m. Sunday in the 100 block of Highland Avenue after the 41-year-old victim got into an argument with the suspect over money, police spokesman Officer Albie Esparza said.
The suspect grew agitated, went and got the bat, then hit the victim in the head with it, Esparza said.
The victim was taken to a hospital to be treated for a cut above his eye and an abrasion to his head, police said. His injuries are not considered life-threatening.
The suspect, a man who is known to the victim and believed to be 26 years old, fled and had not been found as of this morning, Esparza said. He is described as white, about 6 feet tall and weighing 185 pounds, and was last seen wearing a white T-shirt and blue jeans.
Anyone with information about the attack is asked to call the Police Department's anonymous tip line at (415) 575-4444 or to send a tip by text message to TIP411.