Greenwood is in France today to complete his permanent move from Manchester United.
The forward rebuilt his career last season on-loan with Getafe, where he finished their Player of the Year. Greenwood made the move after assault charges were dropped last year.
Barton posted to X: "That is a great signing. This kid deserves a chance to get on with his career.
"We all make mistakes. Nobody’s perfect. England would have won the Euros with this kid in the side.
"He’s as good a player as any in that Euros squad. His behaviour and the public fallout has massively affected his performance, yet he went to an average team in Spain and got himself back on track!"
He also said: "I hope Mason and his family find peace and prosperity in the south of France. Be allowed to get on with their young lives. And hopefully he can get the opportunity to atone for his errors.
"I know the fans of Marseille love their club. This boy could be as good a player as any Englishman that has pulled on your jersey before. With the support your incredible he will set Ligue 1 on fire. This kid is good. Believe me. If you support him, OM take back over.
"Great coach in Roberto De Zerbi. Big conkers to take Greenwood."