A lightvessel, or lightship, is a conventional ship which acts as a lighthouse. It has no means of propulsion and is permanently anchored. It is used in waters that are too deep for a lighthouse, and instead of marking coastlines, usually marks marine traffic routes. It is superior to a buoy because its navigational aids are more visible.

Lightvessels also usually carry data recorders used in oceanography for research purposes, such as wave recorders.

In England and Wales, Trinity House is in charge of all lightvessels. All are now unmanned, but had nine crew in the past. There are 11 lightvessels and 2 smaller lightfloats. The first lightvessel was converted to solar power in 1995, and all except the '20 class' have been solarised. The '20 class' represents a slightly larger type of vessel that derives its power from diesel electric generators and has not been solarised due to navigational requirements. Where a main light with a visible range in excess of 20 miles (37 km) is required a '20 class' vessel is used, as the main light from a Trinity House solar lightvessel has a maximum range of 19 nautical miles (35 km). There are currently hull numbers 19, 22, 23 and 25 (the 20 class), 2, 5, 6, 7, 9, 10, 17 (solar lightvessels) and LF2 and LF3 (solar lightfloats). Solar lightvessels 93 and 95 have recently been decommissioned and scrapped.

The official use of lightships in the United States ended March 29, 1985, when the U.S. Coast Guard decomissioned its last such ship, the Nantucket I. Many lightships were replaced with offshore light platforms (called "Texas Towers") or large navigational buoys - all of which are cheaper to operate, build and maintain than lightships.

"Lightvessel" in other languages: German Feuerschiff, Danish Fyrskib, Swedish Fyrskepp...

See: List of lighthouses and lightvessels, Lightvessels in the United Kingdom...