The Netherlands international joined City from AC Milan in 2025 for 55 million euros, but his time at the Etihad Stadium has come to an end after Al Qadsiah agreed a reported 60 million-euro deal.
Reijnders has signed a four-year contract and is expected to earn more than 100 million euros during his time in Saudi Arabia.
The 28-year-old midfielder was a regular starter for City during the early part of his first season, but his role became less important towards the end of the campaign.
He started only five Premier League matches in 2026 and was an unused substitute in nine of City's final 14 league games. He was also left out of Pep Guardiola's squad for last weekend's Community Shield.
"Two finals at Wembley and a season I'll always look back on. Thank you, City," he posted on Instagram.
Reijnders now joins an Al Qadsiah squad that includes Mateo Retegui, Nacho Fernandez, Julian Quinones, Koen Casteels, Julian Weigl and former FC Utrecht defender Souffian El Karouani.
The club have finished fourth in each of the last two Saudi Pro League seasons and are managed by former Liverpool and Celtic boss Brendan Rodgers, who took charge in December 2025.
Reijnders is the second Netherlands international to move to Saudi Arabia this summer.
Crysencio Summerville, who impressed for the Dutch at the 2026 FIFAWorld Cup, joined Al Hilal from West Ham in a deal worth a reported 64 million euros.
The moves come as the Netherlands begin a new era under Xavi Hernandez, who was appointed head coach following Ronald Koeman's resignation.