It happened around 8:30 p.m. at Trenton Fire Company 1: Engine 1 on the 400 block of Calhoun Street.
Two bullets pierced the back of the firehouse.
Police say one bullet struck the firefighter who was in the weight room. A second bullet went through the wall and into the kitchen, but did not hit anyone.
Authorities say the firefighter was taken to the hospital with a graze wound.
"It's concerning because my firefighters actually have to go to calls with bulletproof vests on. Now to have to be concerned with them to be possibly shot at the firehouse -- this is one of those rare incidents that happen, but this is a major concern," said Ken Douglas, the director of the Trenton Fire Department.
The firefighter was released Tuesday night.
"Nothing made me happier than when I went to the hospital and I ran into the fireman in the parking lot and he was walking, he was fine," said Councilwoman Marge Caldwell-Wilson. "I gave him a big hug."
Police say approximately 12 gunshots were fired about 100 yards away from the firehouse in an apartment complex parking lot.
No arrests have been made at this time.
No other injuries were reported.
"This is just indicative of what's going on across the country with the proliferation of guns which really we all have to pull together to combat," Trenton Mayor Reed Gusciora said.