Sea monsters are sea-dwelling, mythical or legendary creatures, often believed to be of immense size.

Marine monsters can take many forms, including sea dragons, sea serpents, or multi-armed beasts; they can be slimy or scaly, often spouting jets of water. Often they are pictured threatening ships.

Sea monster accounts are found in virtually all cultures that have contact with the sea. Eyewitness accounts come from all over the world. For example, Avienus relates of Carthaginian explorer Himilco's voyage "...there monsters of the deep, and beasts swim amid the slow and sluggishly crawling ships." (lines 117-29 of Ora Maritima). Sir Humphrey Gilbert claimed to have encountered a lion-like monster with "glaring eyes" on his return voyage after formally claiming St. John's, Newfoundland (1583) for England. Another account of an encounter with a sea monster comes from July 1734. Hans Egede, a Danish/Norwegian missionary reported that on a voyage to Gothaab/Nuuk on the western coast of Greenland:

"There appeared a very terrible sea-animal, which raised itself so high above the water, that its head reached above our maintop. It had a long, sharp snout, and blew like a whale, had broad, large flippers, and the body was, as it were, covered with hard skin, and it was very wrinkled and uneven on its skin; moreover, on the lower part it was formed like a snake, and when it went under water again, it cast itself backwards, and in doing so, it raised its tail above the water, a whole ship length from its body. That evening, we had very bad weather."

Other reports are known from the Pacific, Indian and Southern Oceans (e.g. see Heuvelmans 1968).

There is a Tlingit legend about a sea monster named Gunakadeit (Goo-na'-ka-date) who brought prosperity and good luck to a village in crisis, people starving in the home they made for themselves on the Southeast coast of Alaska.