Saint John Abbey, Benedictine nunnery in Müstair, Val Müstair, Switzerland

Saint John Abbey in Müstair, Val Müstair, is a Benedictine nunnery whose core is the Carolingian abbey church begun around 775 as a single-nave hall with three apses and a flat wooden ceiling, later flanked by two side naves to form a five-apse plan. A late Gothic rib vault was inserted by 1492, about a metre below the original ceiling, and the Carolingian frescoes were rediscovered in 1947 and uncovered until 1951. The complex also holds the two-storey Holy Cross Chapel, the double chapel of St Ulrich and St Nicholas, and the Plantaturm.

The monastery was founded around 780 by the bishop of Chur. In 1167 it was turned into a nunnery. Around 960 Bishop Hartpert of Chur built the residential and defensive tower later called Plantaturm, which Abbess Angelina von Planta rebuilt and raised from 1499. The abbess Ursula Carl von Hohenbalken had the Fürstenzimmer built between 1642 and 1659.

The abbey church is dedicated to John the Baptist, while the two-storey Holy Cross Chapel at the cemetery entrance served from the 16th century as an ossuary below and probably as a funerary chapel above. The nuns' choir, built 1488–1492 under Abbess Angelina Planta, has a parapet of cast stucco with whorl rosettes and fish-bladder motifs. The choir stalls designed in 1948 replaced a horseshoe-shaped set made in 1690 for Abbess Lucia Francisca Quadri.

The monastery lies in Müstair in the Münstertal, near the border with Italy's Vinschgau. Since spring 2006, the Stundeweg trail links it with the Abbey of Monte Maria near Mals, a cross-border route co-financed by the European Union's regional development fund. The monastery museum is housed in the Plantaturm, the three-storey residential tower west of the double chapel.

The Carolingian frescoes were whitewashed at some point and forgotten; they were only rediscovered in 1947 and uncovered by 1951. The late-Gothic rib vault built by 1492 sits about a metre lower than the original wooden ceiling, so part of the paintings still lies hidden behind the vault masonry.