Mertens' water monitor (Varanus mertensi), also called commonly Mertens's water monitor, and often misspelled Mertin's water monitor, is a species of lizard in the family Varanidae. The species is endemic to northern Australia, and is a wide-ranging, actively foraging, opportunistic predator of aquatic and riparian habitats. It is named after German herpetologist Robert Mertens.