The Oilliphéist (Irish: ollphéist, from Irish oll 'great', and péist 'worm, fabulous beast, monster, reptile') is a sea serpent-like monster in Irish mythology and folklore.

These monsters were believed to inhabit many lakes and rivers in Ireland and there are many legends of saints and heroes fighting them. In one story, the Oilliphéist cuts the route of the River Shannon when it hears that Saint Patrick has come to drive out it and its kind. In a comic addition to the story, the monster swallows a drunken piper named Ó Ruairc (O'Rourke). The piper is either unaware of his predicament or is completely unperturbed and continues to play inside the Oilliphéist's stomach. The monster becomes so annoyed with Ó Ruairc's music that it coughs him up and spits him out. It is believed by Chris Cairney that this story and one involving Caoránach helped influence and inspire the legend of the Loch Ness Monster.

Other stories of the Oilliphéist exist. One has a girl named Sionnan, grand daughter of Manannán mac Lir, anger the Salmon of Knowledge by throwing stones at it. In revenge the fish summons and asks an Oilliphéist to attack the girl which it does and ultimately ends up killing her.