The 28-year-old joined City last summer in a £46.5m move from AC Milan but struggled to make a consistent impact, making 50 appearances in all competitions and scoring seven goals as Pep Guardiola's side won the FA Cup and Carabao Cup in what proved to be the Spaniard's final season in charge.
He started all three of the Netherlands' group-stage matches at the 2026 World Cup but was an unused substitute in their last-32 defeat by Morocco.
"I have been incredibly privileged to play for Manchester City, and I have relished every minute of my time here," Reijnders said. "From the very moment I arrived, everyone at the club — players, coaches and staff — made me feel so welcome and a part of the Manchester City family.
"I would also like to say a special thank you to the City fans who were so warm and supportive both to me and my family."
Manchester City's new manager, Enzo Maresca, left Reijnders out of his squad entirely for Sunday's disappointing 3-0 Community Shield defeat by Arsenal, a strong signal that his departure was imminent. The Dutch midfielder has signed a five-year contract with Al-Qadsiah, who are managed by former Liverpool and Celtic boss Brendan Rodgers.
Reijnders becomes the latest midfielder to leave City this summer, following Rodri's £65m move to Barcelona and Bernardo Silva's free transfer to Real Madrid after his contract expired.
Spain international Nico González has also been linked with a move to Newcastle, while City remain in advanced talks with Lille over a deal for Morocco midfielder Ayyoub Bouaddi and have not given up hope of signing Chelsea's Enzo Fernández.
The fee represents a small profit for City on the sum they paid AC Milan just over a year ago, when Reijnders arrived off the back of being named Serie A's midfielder of the season. He had signed a five-year contract at the Etihad running until 2030, meaning City are recouping their investment with the vast majority of that deal still to run.
Al-Qadsiah, who finished fourth in the Saudi Pro League last season, have made the move as they continue building towards hosting matches at the 2027 Asian Cup in their new Aramco Stadium.