Semin signed a one-year contract with Canadiens during the off-season, but it was difficult to envision things working out between he and coach Michel Therrien.
The 31-year-old had a goal and four points in 15 games with Montreal.
Therrien made him a healthy scratch eight times during his brief tenure with the team.
Semin will join former NHLers Wojtek Wolski and Oskar Osala (played one NHL game) in Magnitogorsk, as well as Capitals first round pick Ilya Samsonov.
From 2003 to 2015, Semin scored 239 goals and 517 points in 650 games with the Capitals, Hurricanes and Canadiens.