The suspect entered the donut shop at 2542 Alpine around 11 p.m. with a handgun.
He then fled on foot with a small amount of cash. No one was hurt during the robbery.
There was also a robbery at Turtle Tap at 1344 East Grand Avenue in south Beloit. It happened around 7 a.m. Monday morning.
Responding officers are looking for two armed suspects that fled on foot.
They were described as two black men, both wearing masks. No one was injured in the incident.