A hydrofoil is a foil which operates in water. They are similar in appearance and purpose to airfoils.

Hydrofoils can be artificial, such as the rudder or keel on a boat, the diving planes on a submarine, a surfboard fin, or occur naturally, as with fish fins, the flippers of aquatic mammals, the wings of swimming seabirds, or other creatures like the sand dollar.

The term "hydrofoil" is commonly used for the wing-like structure mounted on struts below the hull of a variety of boat (see illustration), which lifts the boat out of the water during forward motion, in order to reduce hull drag. As a synecdoche, the term "hydrofoil" is often used to refer to boat that utilizes hydrofoil technology in this way. Most of this article is about this type of hydrofoil.

As a hydrofoil-equipped watercraft increases in speed, the hydrofoil elements below the hull(s) develop enough lift to raise the hull up and out of the water. This results in a great reduction in hull drag, and a further corresponding increase in speed.

Harry Julius Larsen (September 4, 1915 – August 12, 1974) was a Danish rower who competed in the 1936 Summer Olympics.

In 1936 he won the silver medal with his partner Peter Olsen in the coxless pairs competition.