Trethevy Quoit (Cornish: Koyt Tredhewi)[1] is a well-preserved megalithic structure between St Cleer and Darite in Cornwall, England, United Kingdom.

The special feature of Cornish portal graves is that by such stones sometimes a smaller partially closed area is created before the front end.

However, there would have been a considerable gap between the support stones and the side walls, by which soil could have penetrated into the chamber.

In November 2017 the quoit was added to Historic England's Heritage at Risk Register, because of damage caused by fencing and increasing erosion by livestock.

[7] Hencken in 1932 wrote the first modern interpretation, in which he explained the special nature of the antechamber, and pointed out parallels to structures in Brittany.