Monza - Young Belgian Stoffel Vandoorne will replace Jenson Button as a lead driver with McLaren during the 2017 season, the British veteran said.
Button, 36, is not retiring but said he will be taking a "break" next year.
McLaren followed up the 2009 world champion's announcement by saying they would employ a three-driver strategy next season, with Button signing a two-year contract extension.
But 24-year-old Vandoorne will partner two-time world champion Fernando Alonso on race day.
"McLaren-Honda's race driver line-up next season will be Fernando and Stoffel -- a perfectly balanced mix of proven brilliance and immense potential," said the team's CEO Ron Dennis.
About Button, he added: "Having extended his contract to include 2017 and 2018, he'll continue to be a senior, influential and committed member of the team, and will remain centrally involved in the development of our cars. He'll also be available to race for us if circumstances require it."