Belmont police and firefighters were called to the Leslie Terrace Assisted Living Facility at 2400 Carlmont Drive at about 4:50 p.m.
The 67-year-old woman, who lives at the facility, was found inside the elevator on the second floor, police said.
She told emergency personnel she had entered the elevator on the third floor and as it started to descend, the elevator began to free-fall.
The automatic emergency brake system stopped the car just before the second floor, but the sudden stop threw the woman from her motorized chair onto the floor.
The woman was extricated from the elevator and taken to a hospital, police said. Her injuries were not life-threatening.
The elevator will be out of service as the incident is investigated.