The Val Müstair (German: Münstertal, Italian: Val Monastero) is a mountain valley in the Swiss Alps. It connects the Pass dal Fuorn (Ofenpass, 2149 m) with the Italian province of Bolzano-Bozen and the Vinschgau-Val Venosta (914 meter).
The most important villages in the Val Müstair are: Tschierv (1660 meter), Valchava (1412 meter), Sta. Maria (1375 meter), Müstair (1247 meter) (all Swiss) and Taufers im Münstertal in South Tyrol. There are no other inhabited connected valleys, but the road over the Umbrail Pass (and the Stelvio Pass) is connected with Sta. Maria.
The largest part of the valley is part of the Swiss canton of Graubünden. A small part lies within the region of Trentino-Alto Adige/Südtirol. The border is located at approximately 1245 meter between Müstair (1247 meter) and Taufers-Tubre (1240 meter).
The river in the valley is the Rom (Il Rom or Rombach).