The Spaniard, who took his first IndyCar title with Ganassi in only his second season in the series, made his grand prix weekend debut this year when he drove the McLaren in opening practice for the United States Grand Prix.
He had also tested the MCL35M at Barcelona and the Red Bull Ring as part of his role in the squad's driver development programme.
Palou will compete in IndyCar next year, but will be serving as McLaren's reserve whenever the American series' races don't clash with F1 grands prix. Presently, the series' will clash on 12 occasions in 2023.
"I'm excited to be part of the McLaren team as one of their reserve drivers in 2023," said the 25-year-old Spaniard.
"I've been behind the wheel of both the MCL35M and MCL36 and it has been a great experience, so I can't wait for the involvement with next year's car.
"I look forward to continuing my development as a driver and I appreciate the trust McLaren have in me with this new role next year."
McLaren team principal Andreas Seidl added: "The team are pleased to have Alex on board as one of our reserve drivers for next season. He impressed with his FP1 session in Austin and his Driver Development tests this year.
"He has many notable motorsports achievements so far, particularly the 2021 IndyCar championship title so it's brilliant to be able to expand his role in the team and we look forward to working more closely with him."
Palou will remain with Ganassi for his fourth season in IndyCar. This year, the Catalan driver finished fifth in the standings.
McLaren will have Lando Norris and rookie Oscar Piastri - the 2021 FIA Formula 2 champion - in their Formula 1 line-up next season, the Australian replacing compatriot Daniel Ricciardo alongside the Briton.
Ricciardo will return to reigning F1 world champions Red Bull - with which he raced from 2014 to 2018 - as a reserve driver, having initially been linked with a move to Mercedes in the same role.
