Kubica has been Alfa Romeo’s test and reserve driver since the start of 2020, having been let go by Williams the season prior.
The Pole got his chance earlier this year when Kimi Raikkonen tested positive for coronavirus, forcing him to miss the Dutch and Italian Grands Prix.
Kubica performed solidly in his first two races since 2019 at Zandvoort and Monza, finishing 15th and 14th respectively.
The 2008 Canadian Grand Prix winner will act as back-up for new drivers Valtteri Bottas and Guanyu Zhou.
Along with Kubica’s retention, Polish oil company Orlen will continue as its title sponsor for a third consecutive season.
Speaking of the news, team boss Frederic Vasseur said: “We are delighted to extend our successful partnership with PKN ORLEN into its third year. ORLEN are an instantly recognisable brand in a vast and growing market and they have proven a loyal supporter of the team; their presence underlines and reinforces our resolve to be one of the most attractive teams in the sport for global brands.
“Most importantly, we share a commitment to constant improvement, whether on track or in their business, and we are determined to keep growing together. Their activation campaign has been of the first order, reaching fans all over Europe and expanding access to the team and the world of F1 and we can’t wait to unveil the next stages in this programme.”
Alfa Romeo sits ninth in the F1 constructors’ championship with two rounds to go in 2021.