One person, a 77-year-old woman, was injured in the fire, and several others were forced out into the cold in the 800-block of South Park Terrace.
The fire was first reported at just after 12:30 a.m. It took fire crews about 30 minutes to put out the flames.
The building, called the Oaks of Dearborn Park, is a home for low-income seniors. According to authorities, many elderly residents have been displaced by the fire. It was unclear Tuesday morning exactly how many.
Warming buses were on hand, and representatives from the American Red Cross in Chicago were assisting those who needed their help. They tell ABC7 they have been able to help at least two displaced residents find places to stay.
The injured woman suffered smoke inhalation and was transported to Northwestern Memorial Hospital. Police said she was in stable condition.
No one else was hurt.
The cause of the fire is still under investigation.