Cantre'r Gwaelod was an area of land which, according to legend, was located in an area west of present-day Wales which is now under the waters of Cardigan Bay. Accounts variously suggest the tract of land extended from Bardsey Island to Cardigan or as far south as Ramsey Island.

There are several versions of the myth. The earliest known form of the legend is usually said to appear in the Black Book of Carmarthen, in which the land is referred to as Maes Gwyddno (English: the Plain of Gwyddno). In this version, the land was lost to floods when a well-maiden named Mererid neglected her duties and allowed the well to overflow.

The popular version known today is thought to have been formed from the 17th century onwards. Cantre'r Gwaelod is described as a low-lying land fortified against the sea by a dyke, Sarn Badrig ("Saint Patrick's causeway"), with a series of sluice gates that were opened at low tide to drain the land.

Cantre'r Gwaelod's capital was Caer Wyddno, seat of the ruler Gwyddno Garanhir. Two princes of the realm held charge over the dyke. One of these princes, called Seithenyn, is described in one version as a notorious drunkard and carouser, and it was through his negligence that the sea swept through the open floodgates, ruining the land. The church bells of Cantre'r Gwaelod are said to ring out in times of danger.