Lake monster is the name given to the phenomenon of large animals being sighted and being supposed to exist in lakes, although their existence has never been confirmed scientifically. Sightings are often generally similar to some sea monsters. The study of such creatures is of great interest in Cryptozoology.

The most famous lake monster is certainly the Loch Ness Monster, which for many years has been reported in Loch Ness in Scotland.

A similar monster is Ogopogo that has been sighted in Okanagan Lake in the heart of British Columbia. There are a number of similar monster stories in other North American lakes including Manipogo in Lake Manitoba and Champ in Lake Champlain.

Other locations which have been claimed as homes for lake monsters are Flathead Lake in Montana, Lake Tianchi in China, Bala Lake in Wales, and the White River in Alabama. Fulk's Lake near Churubusco, Indiana is said to be home to the Beast of Busco. The world's deepest lake, Lake Baikal in Siberia has also been cited as a monster's home. Storsjön in Jemtia, Sweden has been said to house Storsjöodjuret.

There are many theories as to what these monsters could be. Many consider them to be based purely on exaggerations and fabrications. The sighting of disturbances in the water caused by seals, fish, logs, mirages, seiches, and light distortion, crossing of boat wakes, or unusual wave patterns have all been proposed. Very large fish have been proposed. Other widely varied theories have been presented, including giant eels and surviving dinosaurs. Some insist they are actually giant sturgeons, while another theory holds that the monsters that are sighted are the occasional full-grown form of an amphibian species that generally stays juvenile all its life like the axolotl. The cryptozoologist Bernard Heuvelmans held throughout his life that pleisiosaur-type sighting were actually unknown species of long-necked seal.

In many of these areas, especially around Loch Ness and the Okanagan Valley , these lake monsters have become important tourist draws.