Rein Abbey (Stift Rein; Latin: Runa) is a Cistercian monastery in Rein near Gratwein, Styria, in Austria. Also known as the "Cradle of Styria" ("Wiege der Steiermark"), it is the oldest surviving Cistercian community in the World.
The monastery was founded in 1129 by Margrave Leopold the Strong of Styria and settled by monks from Ebrach Abbey in Bavaria under the first abbot, Gerlacus. It was the 38th Cistercian monastery to be founded. The previous 37 are all since dissolved, leaving Rein as the oldest extant Cistercian monastery in the world. The abbey has remained a Cistercian community ever since on the same site, except for the temporary exile of a few years during World War II when the premises were confiscated by the Nazis and the monks were evicted until they were able to return in 1945.