Kubica, 35, made his race debut with the team in 2006 when they were competing as BMW Sauber.
The Pole, who suffered a near-fatal rally accident in 2011, made his comeback with struggling former champions Williams last season but left at the end of the campaign in which he scored their sole point.
“I’m starting a new chapter in my career by joining Alfa Romeo Racing ORLEN. This is a team that has a special place in my heart and it’ll be nice to see the familiar faces I still remember from Hinwil,” he said in a statement.
Alfa Romeo’s race drivers this year are Finland’s 2007 world champion Kimi Raikkonen and Italian Antonio Giovinazzi, an unchanged lineup from 2019.