The mast of a sailing vessel is a tall spar, or arrangement of spars, erected vertically or near-vertically on the median line of a ship or boat. A mast may carry sails, spars, and derricks. It may also give necessary height to a navigation light, look-out position, signal yard, control position, radio aerial, or signal lamp. Large ships have several masts, with the size and configuration depending on the style of ship. Nearly all sailing masts are guyed.

Until the mid-19th century, all vessels' masts were made of wood, formed from one or several pieces of timber. This was typically the trunk of a single conifer tree; however, from the 16th century, vessels were often built too large for that. Larger vessels needed taller and thicker masts, which could not be made from single tree trunks. To achieve the required height, these masts were built from up to four sections (also called masts). From lowest to highest, these were called "lower", "top", "topgallant", and "royal" masts. For the lower sections to be thick enough, they needed to be built up from multiple pieces of wood. Such a section was known as a made mast, while a section formed from a single piece of timber was known as a pole mast.

Those who specialised in making masts were known as mastmakers.

For square-sail carrying ships, masts in their standard names in bow to stern (front to back) order, are:

Some names given to masts in ships carrying other types of rig (where the naming is less standardised) are:

When a vessel has two masts, as a general rule, the main mast is the one setting the largest sail. Therefore, in a brig, the forward mast is the foremast and the after mast is the mainmast. In a schooner with two masts, even if the masts are of the same height, the after one usually carries a larger sail (because a longer boom can be used), so the after mast is the mainmast. This contrasts with a ketch or a yawl, where the after mast, and its principal sail, is clearly the smaller of the two, so the terminology is (from forward) mainmast and mizzen. (In a yawl, the term "jigger" is occasionally used for the aftermast.)

Some two-masted luggers have a fore-mast and a mizzen-mast – there is no main-mast. This is because these traditional types used to have three masts, but it was found convenient to dispense with the main-mast and carry larger sails on the remaining masts. This gave more working room, particularly on fishing vessels.

On square-rigged vessels, each mast carries several horizontal yards from which the individual sails are rigged.