Portein is a municipality in the district of Hinterrhein in the Swiss canton of Graubünden.
It is located at the Heinzenberg mountains in the Domleschg region.
On 31 December 2001
, Portein was the smallest municipality in Switzerland in terms of population, with a population of 22. By the end of 2003
, however, the population grew to 23, losing that distinction. Even though the municipality is in a Romansh
-speaking area, it is now almost completely German-speaking.