The first robbery happened just after midnight Tuesday on the 200 block of Avocado Street. The suspect robbed two men as they walked to their car.
The second incident happened just hours later near Adams Avenue and Albatross Road. The victim was confronted by a suspect on a bicycle.
Investigators said while a similar weapon was described, it's unknown if the same suspect was involved in both cases.
Anyone with information on the robberies was asked to call the Costa Mesa Police Department at (714) 754-5205.