The ruins of the Abbey of St Mary, stand at the northern end of Bardsey Island, south-west of the Lleyn Peninsula, in Gwynedd, Wales. The site has had religious importance from at least the 6th century when Saint Cadfan founded an abbey there. In the following centuries the island became an important place of pilgrimage and 20,000 saints are reputedly buried on the island. By the end of the Middle Ages the abbey had declined in importance and, following the Dissolution of the monasteries, fell into ruin. In the 18th century, more substantial remains were still standing, but by the 20th only the current remnant of a tower remained. The ruins are a Grade II* listed building and a scheduled monument. Three Celtic crosses set among the ruins are listed at Grade II.

Saint Cadfan is reputed to have arrived on the island in the 5th century and the following centuries saw it become a place of high importance as a centre for pilgrimage and as a sanctuary for Christians escaping persecution. St Cadfal is said to have built the first abbey on the St Mary's site. By the High Middle Ages the island was reputedly the burial place of some 20,000 saints. It became one of the three Welsh pilgrimage sites of national importance, and Edward I visited in 1284. The Dissolution saw the destruction of the abbey and it subsequently fell into ruins. By the 20th century, only the fragment of one tower remained. Richard Haslam, Julian Orbach and Adam Voelcker, in their 2009 edition Gywnedd, in the Buildings of Wales series, note its setting within an walled enclosure.

The Royal Commission on the Ancient and Historical Monuments of Wales records the dimensions of the "two-stage" tower as 5.8m square with 1.0m thick walls and standing 8.0m high. The ruins are a Grade I listed building, and a Scheduled monument. Three Celtic crosses within the abbey grounds are listed at Grade II.

• Haslam, Richard; Orbach, Julian; Voelcker, Adam (2009). Gwynedd. The Buildings of Wales. New Haven, US and London: Yale University Press. ISBN 978-0-300-14169-6. OCLC 1023292902.

• RCAHMW photographic survey