The first robbery was reported at about 1:35 p.m. near Geary Boulevard and 24th Avenue in the city's Richmond District.
A suspect entered the bank with a gun and ordered employees to get on the ground, according to police.
He then opened the front door for a second suspect while a third suspect entered from a rear door, police said.
The suspects then proceeded to take money from desk drawers and one struck a bank employee in the head, police spokeswoman Officer Tracy Turner said.
A fourth suspect came in briefly to check on the other suspects, and the four eventually fled from the bank on foot. They were described as four black men wearing black hooded sweatshirts and scarves over their faces, Turner said.
The bank employee who was struck was not seriously injured, Turner said.
At about 4:20 p.m., another bank was robbed, this time in the 700 block of Irving Street in the city's Inner Sunset neighborhood.
A lone suspect came into the bank, placed a blank check on the counter and told a teller that he wanted money. He then put his hand in a shoulder bag and simulated that he had a gun, police said.
The teller complied, handing over an undisclosed amount of cash to the suspect, who fled and had not been found as of this morning, police said.
The suspect was described as a white man who was about 6 feet tall, has brown hair and weighs 160 pounds, Turner said.
She said the robberies were not related, but that police are investigating whether the cases have ties to previous robberies involving similar suspects.
Anyone with information about the robberies is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or to send a tip by text message to TIP411 with "SFPD" in the message.