Palou took part in Friday's free practice at this year's US Grand Prix in Austin and has also been involved in McLaren's driver development testing programme.
He will continue to race for Chip Ganassi Racing next season after a failed attempt to move to McLaren's IndyCar team.
McLaren have Britain's Lando Norris (23) and Australian rookie Oscar Piastri (21) as their 2023 Formula One lineup.
"I’m excited to be part of the McLaren team as one of their reserve drivers in 2023. 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," said Palou.
"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 boss Andreas Seidl said Palou had impressed in Austin and other tests.
"It’s brilliant to be able to expand his role in the team and we look forward to working more closely with him," he added.
Eleven of next year's 24 scheduled F1 grand prix clash with IndyCar, whose season starts in Florida on March 5 and ends in California on September 10.
McLaren this year had an agreement to use Mercedes' F1 reserve drivers if needed.
