Underwater minefields (synonyms: naval minefields) are minefields placed under the surface of an ocean or lake. These mines may be free floating or may be chained to the bottom. There may be a means to alter the relative depth of these mines or remotely activate or disarm the explosives. Underwater minefields may be left over from a past war and may be old minefields. They may be combined with ship graveyards or shipwrecks. They may be located in treacherous waters and there may be hidden routes to safely traverse these areas.

When designing underwater minefields consider what region (island, port, trade route, border, supply line, etc.) they were meant to protect. Underwater minefields may have been placed at varied depths depending on if they were planned to thwart boats or submarines. They may be combined with underwater elements like kelp forests or oceanic trenches.