The announcement follows Vandoorne leaving the Mercedes Formula E outfit with which he won the 2021/22 title to join DS Penske, which is part of the same Stellantis automotive group as Peugeot.
The 30-year-old will take the place vacated by British driver James Rossiter, who announced his decision to step down from racing last year to become the team principal of Maserati MSG in FE.
Vandoorne had been handed a chance to sample the Peugeot 9X8 in the Bahrain post-season test last year, but he couldn't make the trip to the Gulf nation after being diagnosed with appendicitis.
He has since made a full recovery and returned to racing last month at the Mexico City E-Prix, which marked the start of FE's Gen3 era.
It's unclear if the Belgian driver got to drive the Peugeot Hypercar over the winter, when the French manufacturer completed a number of tests with the radical 9X8 in preparation for the 2023 season.
With his new role in WEC, Vandoorne will be involved in three different championships this year, having also been announced as a test and reserve driver in F1 for Aston Martin alongside Felipe Drugovich.
"I feel that the programme I have is really the perfect schedule between endurance racing, Formula E and Formula 1," said Vandoorne.
"It's great to be a part of the development of the Peugeot 9X8 Hypercar. The 9X8's unique concept has everybody talking about Peugeot again. We can build a very strong future together."
2023 will mark Peugeot's full campaign in world championship-level sportscar racing in over a decade, having joined mid-way through the 2022 season at Monza.
This means its appearance at June's Le Mans 24 Hours will be its first outing at La Sarthe since 2011, when the leading Peugeot 908 finished second behind the race-winning Audi R18.
The French marque will continue with the same driver line-up in the WEC in 2023 as the end of last year, with Paul di Resta, Mikkel Jensen and Jean-Eric Vergne driving the #93 9X8, and Loic Duval, Gustavo Menezes and Nico Muller sharing the sister #94 entry.
