The 30-year-old has signed a two-year deal with the Swiss Super League side, who are currently fifth after four games in the country's top division.
Song, who left Russian side Rubin Kazan in January, had been training with Sion for several weeks before sealing the move on Tuesday.
The Cameroon international made over 200 appearances for Arsenal between 2006 and 2012 before moving on to the La Liga giants.
Song then returned to the Premier League for two separate loan spells with West Ham, before making the switch to Russia in 2016.
The versatile player has also won 49 caps for the Indomitable Lions before retiring from international duty in 2015.