On a sailing vessel, a forestay is a piece of standing rigging which keeps a mast from falling backwards. It is attached either at the very top of the mast, or in fractional rigs between about 1/8 and 1/4 from the top of the mast. The other end of the forestay is attached to the bow of the boat.

Often a sail is attached to the forestay. One such sail is a jib or a genoa. In a cutter rig, the jib or jibs are flown from stays in front of the forestay, perhaps going from the masthead to the end of the bowsprit. The sail on the forestay is then referred to as the staysail or stays'l.

A forestay might be made from stainless steel wire on a modern yacht and galvanised wire or natural fibers on an older cutter or square-rigged ship.

Contrast with backstay and shrouds.