The forward, who arrived at Reading last year, wanted guaranteed first team football and has moved on to the Swiss side after the Royals agreed an undisclosed fee
The Frenchman appeared 28 times for the Royals, scoring five, but was sent out on loan to Chinese Super League side Shanghai Shenhau in February where he scored two goals in six games.
The 22-year-old has now found a permanent move away from the Madejski Stadium after agreeing a deal with FC Sion, who finished ninth in the Swiss Super League last season, in pursuit of guaranteed first team football.
Manset will join new signings Kyle Lafferty, who spent four years at Rangers ,and former AC Milan midfielder Gennaro Gattuso at the club.
Reading's director of football Nick Hammond told the club’s official website: "Manny scored some important goals for us last season, and so played a part in helping us reach the Premier League.
"However he wants regular first team football and we can't guarantee that this year; we'd like to wish him the best of luck in his future career."