St Breock Downs Monolith (or St Breock Longstone; Cornish: Men Gurta^[1]) is the largest and heaviest prehistoric standing stone in Cornwall, England.^[2] It stands on the summit of St Breock Downs.

The stone is made from the local Devonian shale which has extensive feldspar veining,^[3] and it is estimated to weigh around 16.5 tonnes.^[1] It is 4.92 metres long^[1] and stands to a height of just over 3 metres above ground level.^[3] It stands on a low stone mound or cairn with a diameter of around 10 metres.^[3] It is believed to be Late Neolithic or Early Bronze Age (around 2500-1500 BC).^[1]

It fell over in 1945, and was re-erected in 1956 after a small excavation had been carried out.^[3] The excavation showed that the stone stood in a setting of quartz pebbles below which were two small hollows.^[1] Similar hollows at other sites have been found to contain human bone or ashes.^[1]

The stone may have been associated with other Bronze Age ritual monuments in the area, including one other standing stone,^[4] and a series of barrows that extend up to 4 miles (7 km) to the west.^[3]

The stone is mentioned in antiquarian records as early as 1613, and was later adopted as a St Breock parish boundary marker.^[3] The site is now in the care of the Cornwall Heritage Trust on behalf of English Heritage.^[5]