The fire burned in the southeastern area of the U.S. Army Reserve Parks Reserve Forces Training Area began around 1:40 p.m. and was contained by 4:30 p.m., spokeswoman Susan Clizbe said.
A smoke grenade used during an outdoor military training sparked the fire, she said.
The area is north of Interstate Highway 580 and east of Interstate Highway 680.
No structures were affected and no injuries had been reported, she said.
The Camp Parks Fire Department is responding to the fire, along with the Alameda County Fire Department and Cal Fire, she said.