Fishing village and harbour on the southern coast of the Isle of Man, in the parish of Kirk Christ Rushen.

The settlement appears in earlier sources as Port le Marie and similar forms. Robertson's list of Manx ports and creeks places Port le Marie among the creeks belonging to Derbyhaven, together with Castletown harbour, Poolvash, Port Erin, Fleshwick and Niarbyl.

Fishing was the principal occupation of the village. Port St Mary developed as one of the southern stations of the Manx herring fleet and later supported a substantial fishing community working the waters around the Calf, Spanish Head and the southern coast.

The harbour was naturally sheltered but limited in size, and its improvement became a recurring concern as fishing boats increased in number and size. Quays, breakwaters and harbour works gradually altered the original shore. The same pressures affected Port Erin on the opposite side of the southern peninsula, although the two ports faced different waters and landing conditions.

The surrounding coastline contained older religious and settlement sites. Chapel Bay preserves the name of a chapel close to the shore, while the wider parish of Rushen included Port Erin, Spanish Head, the Calf of Man and the agricultural land inland from both ports.

Port St Mary remained closely tied to the fishing industry into the modern period. Later accounts of Manx sea fishing regularly list it alongside Peel, Ramsey and Port Erin as one of the Island's principal fishing stations.