The Loch Ness Monster (Scottish Gaelic: Niseag) is a cryptid that is reputed to inhabit Loch Ness in the Scottish Highlands. It is similar to other supposed lake monsters in Scotland and elsewhere, though its description varies from one account to the next. Popular interest and belief in the animal has varied since it was brought to the world's attention in 1933. Evidence of its existence is anecdotal, with minimal and much-disputed photographic material and sonar readings.

The most common speculation among believers is that the creature represents a line of long-surviving plesiosaurs. The scientific community regards the Loch Ness Monster as a modern-day myth, and explains sightings as a mix of hoaxes and wishful thinking. Despite this, it remains one of the most famous examples of cryptozoology. The legendary monster has been affectionately referred to by the nickname Nessie (Scottish Gaelic: Niseag since the 1950s.