Everton announced on Tuesday that the France midfielder had completed a move to Ligue 1 side Nice.
No terms of the deal were announced.
Schneiderlin, 30, made 88 appearances across four seasons for the Toffees since a £20 million transfer from Manchester United in 2017.
Prior to joining United in 2015, Schneiderlin spent seven seasons at Southampton, playing with Saints in League One, the Championship and the Premier League.
The native of Obernai returns to Ligue 1 for the first time since 2008 when he was a member of Strasbourg.
Internationally, Schneiderlin has been capped 14 times by Les Bleus.