The one-alarm fire was reported at about 2:45 a.m. in the shed, which is attached to a two-story apartment building at 1201 Sanford Ave., Pinole Fire Battalion Chief Brian Lowry said.
While firefighters were putting out the fire, they found the dog, a pit bull that was unresponsive in the shed, Lowry said.
Firefighters used an oxygen mask to successfully resuscitate the animal and treat it for smoke inhalation, Lowry said. The dog was still alive this morning but veterinarians do not know if it will survive, he said.
Fire officials have been unable to find the dog's owner, who is not believed to be a resident of the apartments, Lowry said. The animal is currently in the custody of the Contra Costa County Animal Services Department.
No one was injured by the fire, which was extinguished within 10 minutes. The blaze caused about $5,000 in damage to the shed and its cause is under investigation, fire officials said.