Barkley was introduced to his new side's supporters before kick-off and had previously been linked to Southampton, Rangers and Celtic before signing with the Ligue 1 side.
"A British blend of talent, technique and aggression topped off with character, the 28-year-old has decided to continue his career at Le Gym," read a club statement.
"Though the summer transfer window closed last Thursday evening in France, he can join the Aiglons as he was out of contract. Welcome Ross!"
Barkley made 100 appearances for Chelsea in all competitions, scoring 12 goals.
Barkley's best moment at the Blues came in his substitute appearance in the 4-1 Europa League final triumph over Arsenal in Baku in 2019.
A product of Everton's academy, the Liverpudlian graduated to the first team in 2010 where he stayed, with the exception of loan spells to Sheffield Wednesday and Leeds, until signing with the Blues in 2018.
Chelsea agreed a deal to settle the remainder of the England midfielder's contract, allowing the 28-year-old to leave Stamford Bridge as a free agent.
Barkley made his Premier League debut aged 18, and was called up by England before his 20th birthday.
He has made 33 appearances for the Three Lions, including the 2014 World Cup and the 2019 Nations League, scoring six goals.
Wales international Aaron Ramsey, former Leicester goalkeeper Kasper Schmeichel, ex-Manchester United, Everton and Southampton midfielder Morgan Schneiderlin and on-loan trio Nicolas Pepe, Joe Bryan and Mads Bech Sorensen are others with Premier League experience who are also currently at Nice.