The fire at Roy's Redwoods Open Space Preserve just north of the San Geronimo Golf Course near Woodacre was reported around 7 a.m., Marin County spokesman Brent Ainsworth said.
Crews set up a staging area in a nearby meadow and approached the center of the fire from several angles, Ainsworth said.
Cloud cover prevented an air attack on the fire, which was 50 percent contained around 10:45 a.m., Ainsworth said.
Marin County Fire Department Battalion Chief Mike Giannini said 85 firefighters responded to the blaze in the old growth redwoods that are among the largest in the county.
Ainsworth said several fire crews have been released and Roy's Redwoods Open Space Preserve will be closed for 48 hours to fight hot spots and investigate the cause of the blaze.
Marin County Fire Marshal Scott Alber said the fire appears to be human-caused, but investigators do not know if it is accidental or intentional.
No structures were threatened in the area and no injuries were reported.
Giannini said the fire spread quickly even in the damp and humid morning hours.
"This exemplifies the potential for high fire activity as a result of the continuing drought," Giannini said.
Roy's Redwoods is managed by the Marin County Open Space District. The 293-acre preserve is a few miles east of where 40 acres burned in June on Mount Barnabe inside Samuel P. Taylor State Park, Ainsworth said.
Roy's Redwoods is also about nine miles northwest of Muir Woods National Monument and 8.5 miles northwest of downtown San Rafael.