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.
The announcement was made by Ferrari chairman Sergio Marchionne on Saturday.
The white-and-red scheme, reflecting Alfa Romeo's colours, marks a move away from the predominantly blue livery which Sauber has run since 2015. Even before he put his teenage years behind him back in October, he had always been the favorite to get this ride, and now it's official.
With this confirmation, just one seat remains on the F1 grid for 2018 as Williams continues to consider who will replace Felipe Massa and partner Lance Stroll next year.
Giovinazzi will be the reserve driver instead, taking part in Friday practice sessions.
Fiat Chrysler have invested billions of euros since 2014 to develop new Alfa Romeo models that would stand up against German premium rivals and shed a reputation for poor quality and service that has hit sales in the past decade.
"The motor we use will be the same one Ferrari use", Marchionne said. "I think the guns are loaded in a different way".
The 38-year-old Raikkonen signed a third straight one-year deal to remain with Ferrari through 2018, but a return in 2019 on a fourth consecutive one-year deal isn't overwhelmingly likely. "First and foremost, I would like to thank Ferrari for their support".
Ferrari's late founder Enzo also started out racing and managing a team for Alfa Romeo, before setting up on his own in the late 1930s.