Jenson Button will return to Formula 1 for the Monaco Grand Prix, standing in for McLaren's Fernando Alonso.
The 2009 world champion will partner Stoffel Vandoorne in next month's race, while Alonso makes his debut in the Indianapolis 500.
Button started 305 grands prix, including 170 with McLaren, before stepping aside at the end of the 2016 season.
He remains on McLaren's books, with a contractual possibility of returning to racing in 2018 after this year's sabbatical, and has been drafted in for Monaco.
"I'm thrilled to be making a one-off return to Formula 1 racing, and I couldn't think of a better place to make that return than my adopted home Grand Prix: Monaco," he said.
"I've won the race before, in 2009, and it's one of my all-time favourite racetracks.
"It's a tricky street circuit on which a good driver can really make a difference - and, although the McLaren-Honda MCL32 hasn't begun the season well, I think it may be more suited to Monaco than to the faster circuits that Fernando and Stoffel have raced it on so far this season.
"OK, I realise we won't have a realistic chance of repeating my 2009 victory, but I think we'll have a opportunity to score world championship points, which will be very valuable to the team in terms of constructors' rankings.
"I'll drive the MCL32 around Monaco in the McLaren sim beforehand, and I reckon I'll be ready for the race after doing that.
"I'm supremely fit, having done a lot of triathlon training recently, so I have no worries on that score."
Hours after Alonso's surprise Indy 500 bid was announced on Wednesday, McLaren executive director Zak Brown said the team had not locked in a replacement.
However racing director Eric Boullier said Button was a no-brainer, with Alonso to return from the following Canadian Grand Prix.
"I was truly delighted when Jenson accepted our suggestion that he race at Monaco instead of Fernando," he said.
"Jenson is a class act. He's a superb driver - fast, smooth and precise - and he won't have lost any of his competitive edge over the past few months.
"After all, he's missed only a handful of grands prix since his last outing in Abu Dhabi in late November last year, and he's as fit as a fiddle.
"Also, he's always been good at Monaco. He'll do a great job for us, I'm sure of that."