The Algerian midfielder had an agreement with AC Milan running until 30 June 2027, but the two parties agreed to terminate it early, freeing him to sign for Al-Gharafa in the top flight of Qatari football.
The Rossoneri confirmed the move in a brief official statement: "AC Milan confirms the mutual termination of Ismaël Bennacer's contract. The club wishes to part ways with the Algerian player and would like to wish him all the best for the future."
Bennacer leaves having made 178 appearances and scored eight goals for Milan, along with a run to the Champions League semi-final in 2023.
Bennacer, who joined Milan from Empoli in the summer of 2019 and went on to become a key figure in the club's Scudetto-winning side of 2021/22, chose to mark the end of his six-year spell with a lengthy farewell message posted on Instagram rather than simply letting the club statement speak for him.
"Hello Milan fans, last year I said goodbye to you without imagining that those words would end up meaning a real farewell," he wrote.
"This can't erase the memories built here over more than five years. I wore this shirt with pride, passion and determination alongside extraordinary teammates and supported by your unconditional love.
"Winning the Scudetto in this shirt will always remain one of the most beautiful memories of my career. Playing at San Siro, in front of you, was also one of the greatest honours."
He went on to thank those who had supported him throughout his time in Milan.
"I want to thank everyone who has been with me on this journey: my teammates, the various staff, the club's employees and everyone who helped make this adventure so special. A special thought goes to you, Rossoneri fans, whose love and support will forever remain etched in my heart.
"You are what makes Milan a unique club. I hope our paths can cross again, in one way or another, because deep down I feel our story isn't quite finished yet. Thank you for everything.
"Forza Milan forever."