A keel is a structural element on a watercraft that runs along the centerline of the ship, from the bow to the stern. It is the bottom-most longitudinal structural member around which the hull of a ship is built. The keel is often the first part of a ships hull to be constructed, and laying the keel, or placing the keel in the cradle in which the ship will be built, may mark the start time of its construction.

The keel serves two purposes in sailboats: as an underwater foil to minimize the lateral motion of the vessel under sail (leeway) and as a counterweight to the lateral force of the wind on the sail(s) that causes rolling to the side (heeling). As an underwater foil, a keel uses the forward motion of the boat to generate lift to counteract the leeward force of the wind.

There are different types of keels, including the flat plate keel, which is the most common type and is fitted in the majority of ocean-going ships and other vessels. Another type is the bar keel, which may be fitted in trawlers, tugs, and smaller ferries. In sailboats, keels come in many variants, with perhaps the most common being non-sailing and sailboat keel.

The importance of the keel is to help give the boat greater stability and control while moving forward. Without the keel, a boat might slip or skim on the water, which is often common in vessels with an entirely flat bottom. The keel is a flat blade sticking down into the water from a sailboats bottom that prevents the boat from being blown sideways, and it holds the ballast. Its primary purpose is to keep the vessel upright.