A lifebuoy or life ring, among many other names (see § Other names), is a life-saving buoy designed to be thrown to a person in water to provide buoyancy and prevent drowning. Some modern lifebuoys are fitted with one or more seawater-activated lights to aid rescue at night.

Other names for "lifebuoy" include:

The lifebuoy is usually a ring- or horseshoe-shaped personal flotation device with a connecting line allowing the casualty to be pulled to the rescuer in a boat. They are carried by ships and boats and located beside bodies of water and swimming pools.

In the United States, Coast Guard approved lifebuoys are considered Type IV personal flotation devices. At least one Type IV PFD is required on all vessels 26 feet or more in length.

In the UK the Royal Life Saving Society considers lifebuoys unsuitable for use in swimming pools because throwing one into a busy pool could injure the casualty or other pool users. In these locations, lifebuoys have been superseded by devices such as the torpedo buoy, a low-drag device developed to be towed by lifeguards to those in danger.

Leonardo da Vinci sketched a concept for a safety wheel, as well as for buoyant shoes and balancing sticks for walking on water.

According to various sources the Knights of Malta were the first to use cork lifebuoys on their ships.

In the book Architectura naval antigua y moderna (1752) by Juan José Navarro, 1st Marquess of Victoria, two plates show "circular lifebuoys" and another plate includes a drawing of "a lifebuoy made of cork", called "salvenos". This is the type used systematically by the Knights of Malta on their ships. The lifebuoy was attached to a rope on one side and to the poop of the ship on the other, so that it may be deployed in case anyone should fall into the sea. Navarro was Captain General of the Navy and is credited with the systematic introduction of the lifebuoy on all ships of the Spanish navy.