The fire was reported around 9:15 a.m. Friday at the popular Philadelphia cheesesteak shop on the 400 block of South Street.
Smoke continued to pour out of the building into the early afternoon.
It was finally declared under control at 1:33 p.m.
Chopper 6 over fire at Jim's Steaks on South Street
The fire alarm reached two alarms as more crews were called in. Officials say it was a difficult battle because the flames were moving through the heating and air conditioning system.
Christina Lawlor was in the building Friday morning, opening Jim's Steaks for the day.
"I knew it when I walked in this morning something wasn't right because it was too hot," she said.
"I started smelling something. It smelled electrical, you know you can smell that," she continued. "So I'm like, 'something's not right.' We looked up and saw smoke coming down from where the walk-in is and it was smoke pouring down."
In all, the fire department says more than 125 personnel responded to the scene, along with nearly 60 vehicles. Companies will remain there to douse hot spots.
The fire marshal's office will investigate the cause, the department said.
"There are fire trucks almost at every block in the neighborhood," South Philadelphia resident Nicole Wiegand said. "It seems like an all-hands on deck situation, which is kind of scary."
"I feel really sad for our neighborhood. It's like our little corner of the world can't catch a break lately, and I just hope everyone continues to support all the businesses that are here as best as they can," resident Nicole Krecicki said.
Chopper 6 over the scene showed crews using ladders and spraying water on all sides of the building, front and back.
At one point, firefighters could be seen on the second floor of the building breaking windows.
Chopper 6 over fire at Jim's Steaks in South Philadelphia
No injuries have been reported. The people inside the building were already evacuating the building by the time firefighters arrived, Commissioner Adam Thiel said during a news conference.
There are no apartments above Jim's, Thiel said. All of the space above the restaurant is used for storage.
News conference on blaze at Jim's Steaks on South Street
The building next door is also commercial property, Thiel said.
Jim's Steaks South St. just celebrated its 46th year earlier this month, opening its doors for the first time in 1976.