Raikkonen signed a two-year contract with Sauber to race with them from next year.
The Finn will swap seats with Charles Leclerc, who heads to Ferrari, for the 2019 season. However, Raikkonen will be joined by a new face in Antonio Giovinazzi, who was a reserve driver for Ferrari.
Giovinazzi has twice previously raced for Sauber at the start of 2017 as a stand-in for Pascal Wehrlein.
Ericsson will remain with the team as a third-choice back-up driver after a disappointing campaign where he has scored just six points – the third lowest on the grid.
“I am thankful for the past five years of my career which I have spent racing in Formula 1,” said Ericsson.
“I am proud to have represented Sauber for four of those years, and for having raced for such an iconic brand as Alfa Romeo this season.
“I would have loved to continue on this path, but having a driver like Kimi Raikkonen on board is a great opportunity for the whole team.
“Going forward, I will continue contributing to the development of the team as I have always done, with full dedication and all of my support.”
Sauber team principal Frederic Vasseur thanked Ericsson for his efforts.
“We would like to thank Marcus for his dedication to the team and for the great job he has done over the past four years,” said Vasseur.
“We are very pleased that he will continue being part of the Sauber family in the future.
“In 2018, the team has made important progress and Marcus’ work has been essential to our development.
“He has a long history with our team, and we look forward to our new shared chapter.
“Together, we will strive to continue moving in this positive direction.”