A merchant ship, merchant vessel, trading vessel, or merchantman is a watercraft that transports cargo or carries passengers for hire. This is in contrast to pleasure craft, which are used for personal recreation, and naval ships, which are used for military purposes.

They come in myriad sizes and shapes, from six-metre (20 ft) inflatable dive boats in Hawaii, to 5,000-passenger casino vessels on the Mississippi River, to tugboats plying New York Harbor, to 300-metre (1,000 ft) oil tankers and container ships at major ports, to passenger-carrying submarines in the Caribbean.

Many merchant ships operate under a "flag of convenience" from a country other than the home of the vessel's owners, such as Liberia and Panama, which have more favorable maritime laws than other countries.

The Greek merchant marine is the largest in the world. Today, the Greek fleet accounts for some 16 per cent of the world's tonnage; this makes it currently the largest single international merchant fleet in the world, albeit not the largest in history.

During wars, merchant ships may be used as auxiliaries to the navies of their respective countries, and are called upon to deliver military personnel and materiel.