Rein Abbey was a Roman Catholic religious house for women located in Rissa on the Fosen peninsula to the northwest of Trondheim, in Sør-Trøndelag, Norway.
The abbey, built on a prominent elevation in an otherwise flat landscape, and dedicated to Saint Andrew, was founded in or shortly after 1226 on his ancestral estate, on which there already stood a castle, by Duke Skule Bårdsson, apparently in fulfilment of a vow after recovering from an illness. The first abbess was Duke Skule's half-sister, Sigrid Bårdsdatter; his daughter, Margret of Norway, queen of Håkon Håkonsson, spent her last years there; and many other women of the aristocracy entered it. While there is no definite information on what order, if any, it belonged to, it seems to have been a collegiate foundation, or house of secular canonesses, for noblewomen.
The buildings were struck by lightning and burnt down in 1317, but quickly repaired.
During the Reformation the abbey was dissolved and its assets taken over by the Crown. In 1531 the powerful and wealthy Ingerd Ottesdatter Rømer, otherwise Ingrid til Austrått, a leader of the Norwegian aristocracy, had herself elected administrator of the abbey, and was thus able to protect the abbess and canonesses; the abbey's estates also became hers, and continued in the possession of her descendants.
Some remains of the abbey structures are still to be seen among later buildings, but the site has only once been investigated archaeologically, in 1861.
Coordinates: 63°33′40″N 9°56′00″E / 63.56111°N 9.93333°E