Those injured include four firefighters and five civilians.
Two of the injured civilians had to be flown to a burn center. There was no immediate word on their conditions.
The firefighters suffered non-life-threatening injuries.
Both the boarding house and a home next door were destroyed. City officials said in a news release that a total of 76 people were displaced.
The fire was reported around 7:53 a.m. in the unit block of S. Georgia Ave.
Firefighters battling raging blaze at building in Atlantic City, New Jersey
Shydia Black lives in the home that was destroyed. She was in her car when the fire began.
"I'm screaming, trying to get to my children," Black said. Her family members, including the children, got out safely.
Video recorded by Black shows a person falling from the boarding house while escaping the flames.
The person is seen hanging from a fourth-story window for a few moments before falling.
"I watched so many people get hurt," said Black said. "I keep hearing their screams in my ears and there was nothing I could do."
The Red Cross set up an assistance center at Boardwalk Hall for the people who were displaced.
"The city worked with some of their departments. They were able to find lodging and housing for all these people," said Jim Eden with the Red Cross.
The fire was placed under control at 1:10 p.m.
There has been no word on the cause of the blaze.