The robbery was reported at about 1 a.m. at the Black Bear Diner, located at 700 Bancroft Road, according to police.
Three suspects, all wearing masks and all-black clothing, overtook an employee exiting the rear of the restaurant after it had closed for the night.
One suspect struck the employee in the face while the other two entered the restaurant and ordered the only other employee inside to open the cash register, police said.
One of the suspects was armed with a 12-inch knife, according to police.
After taking cash from the register, the suspects moved both employees into an office at the restaurant and ordered them to open the safe in there.
The suspects took about $600 in total before fleeing the restaurant on foot, police said. Officers responded and searched the area but did not find the suspects.
The suspect armed with the knife was described as being about 5 feet 4 inches tall with a medium build. The second suspect is about 5 feet 8 inches tall with a thin build, while the third was only described as carrying a small bat, according to police.
Anyone with information about the robbery should contact Walnut Creek police at (925) 943-5844.