The first fire was reported shortly before 4 p.m. in the 1600 block of Slatten Ranch Road and charred 20 acres, according to Contra Costa County Fire Protection District spokesman Steve Aubert.
Fire crews responded about 4:30 p.m. to the second, smaller fire near the Antioch Marina that burned about 5 acres.
Both blazes were contained and cleared shortly before 8 p.m., Aubert said.
No injuries were reported in the fires and no buildings were threatened.
The cause for both fires is not known.