The rescuer – a regular sea swimmer – brought the person to the shore as the Fire and Rescue Service inshore boat and a fire engine were sent to the scene at about 20:00.
In a statement, the service said that “it was clear that without the assistance of the member of the public, the outcome may have been very different”.
The swimmer and the person they saved were attended to by paramedics at the scene.
The Fire and Rescue Service said: “The member of the public who swam out to help without doubt helped saved the life of the casualty and Jersey Fire and Rescue would like to commend his actions.
“[He] is a regular sea swimmer, and familiar with the local area. We would however advise members of the public not to put themselves in danger.
“If you find yourself in an emergency or spot someone else in trouble, you should call 999 and ask for the Coastguard.”