Hirakawa will become part of McLaren's driver development programme, supporting the Woking team in its simulator as well as conducting a testing programme with its 2021 MCL35M car.
The 29-year-old will dovetail his F1 testing role with his current drive for Toyota, with which he is currently leading the Hypercar class in the World Endurance Championship.
Hirakawa and his team-mates Sebastien Buemi and Brendon Hartley are the reigning champions too, having won both the overall title and the Le Mans 24 Hours in 2022.
“I’m happy to have joined the McLaren F1 team as a reserve driver," Hirakawa said on the margins of this weekend's Japanese Grand Prix.
"It’s an exciting prospect to work with such a respected team and I’m grateful to Zak [Brown] and Andrea [Stella] for the opportunity.
"I’ve already met the team and experienced the simulator at MTC which has allowed me to hit the ground running. I’m now fully prepared and focused for the remainder of 2023.
"I am grateful to [Toyota Gazoo Racing] for allowing me to serve as a driver for the two teams. The second thank you goes to Morizo-san [Toyota chairman Akio Toyoda], who has supported me as a driver."
Team principal Stella added: “I’m pleased to welcome Ryo to McLaren Racing as a Formula 1 reserve driver to further strengthen our growing support driver pool.
"We are thankful to Toyota Gazoo Racing for their collaboration by allowing Ryo to take on the new role alongside his driving duties. He has an impressive record on track winning Le Mans as well as the FIA World Endurance Championship.
“We look forward to working closely with Ryo who has already spent time on the simulator at MTC. His wealth of experience will benefit the team in a number of areas as he joins our strong stable of drivers."
Though a left-field appointment, Hirakawa has single-seater experience as well, as he currently races in Japan's Super Formula championship. He was runner-up in the series in 2020.
The Toyota protege also previously raced in SUPER GT, taking the crown for its luxury brand Lexus in 2017. He is also a former champion of Japan's F3 championship.
McLaren had an agreement to share Aston Martin's reserve drivers Stoffel Vandoorne and Felipe Drugovich for the first 15 rounds of 2023 and also has Mercedes' Mick Schumacher in its reserve pool.
It previously also had two-time IndyCar champion Alex Palou on its books before his deal to join its IndyCar arm from Chip Ganassi Racing collapsed. It is instead working on adding its current IndyCar star Pato O'Ward to its F1 reserve pool for 2024, pending the Mexican acquiring a superlicence.
