Officers received a report of an armed robbery at a Valero gas station at 14660 Monterey Road at 1:19 a.m.
They arrived and spoke with a female clerk, who said a masked man had approached the counter, pointed a semi-automatic handgun at her and demanded money, police said.
She gave him cash from the register and he fled, running south on Monterey Road. The clerk was not injured.
The robber was described as a white or Hispanic man about 23 years old, 5 feet 8 inches tall, and 150 to 160 pounds.
He was wearing a black sweatshirt, black shoes, black pants, black gloves and a black mask over part of his face, police said.
Anyone with more information is asked to call Morgan Hill police Detective Brandon Richards at (408) 779-2101 or the anonymous tip line at (408) 947-STOP (7867).