A security guard who spotted a woman in the water at Leeds Dock near the Royal Armouries, held onto her until fire and rescue crews arrived.
Crews from Hunslet, Rothwell and Brighouse were called the the scene, on Armley Drive at 12.45am.
A spokeswoman for West Yorkshire Fire & Rescue Service said: “The woman was seen on CCTV as being in the water. A security guard held onto her until the fire service arrived.”
Firefighters were able to rescue the woman using a roof ladder and she was taken to hospital by ambulance.