The 25-year-old Spaniard will act as back-up to Lando Norris, and his new rookie team-mate Oscar Piastri, at Formula One races that do not clash with his IndyCar schedule.
Palou took part in practice for McLaren at the United States Grand Prix in October, and won the 2021 IndyCar title.
"I'm excited to be part of the McLaren team as one of their reserve drivers in 2023," said Palou.
"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. It's brilliant to be able to expand his role in the team and we look forward to working more closely with him."