A walker spotted the man in his 30s in the water at 9.45am and threw a life ring to him before calling Lancashire Police and the North West Ambulance Service.
The man had gone into the water near Chiquito on Port Way in Preston.
The man climbed out of the water and is now being treated by paramedics although his condition isn't thought to be serious.
A police spokesman said: "We were called to the incident at around 9.45am this morning after a man in his 30s had been seen in the water at the docks.
He was seen near to the Ribble Pilot pub on Mariner's Way and a passerby has thrown him a life ring. He has then climbed out and is now being treated by paramedics.