Monnikenlangenoog (Dutch pronunciation: [ˈmɔnɪkə(n)ˌlɑŋənˌoːx]; English: Monks' Long Island), also called Moenkelangenoe, was a West Frisian island in the Wadden Sea. It was situated off the coast of present-day Groningen in the Netherlands, between the islands of Schiermonnikoog and Borkum. Monnikenlangenoog was 12 to 14 or 15 km long. During summers, the island was used for animal farming, which was of economic importance. In the 14th century, the island was the property of the St. Juliana's Abbey in Rottum and the Old Abbey in Kloosterburen, both on the mainland.