Casti-Wergenstein or Casti-Vargistagn is a former municipality in the Viamala Region in the Swiss canton of Graubünden. On 1 January 2021 the former municipalities of Casti-Wergenstein, Donat, Lohn and Mathon merged to form the new municipality of Muntogna da Schons .

Casti-Wergenstein was created in 1923 through the union of the formerly independent municipalities of Casti and Wergenstein.

Casti is first mentioned in 1204 as ad Castellum. Wergenstein is first mentioned in 1219 as Vergasteno.

Casti-Wergenstein had an area, (as of the 2004/09 survey) of 25.62 km2 (9.89 sq mi).