According to transfer journalist Fabrizio Romano, the two clubs are in advanced negotiations and closing in on an agreement, with Reijnders' own approval now the final piece needed to complete the move.
Manchester City signed the 28-year-old from AC Milan last summer for a fee reported at around €55 million, and any sale to Al-Qadsiah would represent a smaller loss than initially feared given the level of interest already circling the midfielder.
Reijnders started life at the Etihad Stadium strongly but saw his role diminish as the season progressed, finishing with five league goals and two assists from 28 Premier League appearances, 19 of them starts.
He holds a contract with City running until the summer of 2030, giving the club no financial pressure to sell, though manager Enzo Maresca has been reshaping his midfield options since taking charge and Reijnders is no longer considered one of the untouchable names in the squad.
Nottingham Forest and Juventus have both been credited with interest in the Netherlands international earlier in the window, and Romano reports that Forest remain keen even as talks with Al-Qadsiah progress, leaving the final decision in Reijnders' hands rather than the clubs'.
Should the Saudi move go through, Reijnders would follow compatriot Crysencio Summerville, who completed a switch to the Saudi Pro League earlier this summer, continuing a growing trend of established Eredivisie-honed talent moving to the Gulf.
Reijnders previously spent time at AZ Alkmaar before establishing himself at Milan, where his form persuaded City to move for him last year.
His potential departure would mark a striking reversal for a player signed as a long-term midfield building block barely twelve months ago, though for now the transfer remains dependent on Reijnders giving his personal green light to a move that would take him a considerable distance from the competitive stage he has spent his career building toward.