The 20-year-old from Monaco, a member of the Ferrari Driver Academy, will take Pascal Wehrlein's place behind the wheel next season as Sauber team up with Alfa Romeo.
F2 champion LeClerc had always been expected to make the jump to the pinnacle of motorsport. While Sauber is not a competitive team and Leclerc will likely struggle to score points by finishing races in the top 10, this will be a great opportunity for him to showcase his driving talent to some of the sport's top teams.
The announcement was made by Fiat Chrysler (FCA) chief executive Sergio Marchionne on Saturday at a presentation of Alfa Romeo's new partnership with the Swiss team at the Italian marque's museum near Milan. However, Ericsson's close ties to Sauber owners Longbow Finance appear to have edged it.
As for Ericsson, the Swede was in a battle with Antonio Giovinazzi for the second seat before Sauber opted to stick with what they know.
Leclerc has risen rapidly through the single-seater ranks, winning the GP3 title previous year before his F1 championship triumph this season. A plan to run Honda engines in 2018 was cancelled by new boss Frederic Vasseur.
"I would also like to thank Sauber for their trust and confidence in me as a driver, and look forward to joining the Alfa Romeo Sauber F1 Team for the upcoming season". The title sponsorship deal means the team will be called Alfa Romeo Sauber F1 and the multi-year partnership comes with technical and commercial benefits.