Kubica made his return to the F1 grid with Williams in 2019 following a long absence triggered by his life-threatening rally crash eight years prior.
The 35-year-old scored Williams' sole point in what proved a miserable season for the Grove-based team, and made way for Nicholas Latifi in Williams' driver line-up for 2020.
Kubica was instead courted by the likes of Haas and Racing Point for a simulator role, but will link up with Alfa Romeo instead.
His new role comes against the backdrop of "a multi-year partnership" between the team and Polish oil company PKN Orlen, which will see the Swiss-based outfit compete as 'Alfa Romeo Racing ORLEN' next year.
"I am really happy to be starting this new chapter in my career as I join Alfa Romeo Racing ORLEN," Kubica said.
"This team holds a special place in my heart and I am pleased to see some faces still here from my years in Hinwil.
"Time and circumstances are obviously different, but I am convinced that I will find the same determination and hunger to succeed. I am looking forward to helping Alfa Romeo Racing ORLEN make the next step forward."
Kubica drove for the Peter Sauber-founded F1 outfit between 2007 and 2009, when it competed with factory backing from German manufacturer BMW.
During that time, he won the 2008 Canadian Grand Prix and finished in a personal best fourth place in the drivers' championship.
The team retained an unchanged line-up of Kimi Raikkonen and Antonio Giovinazzi for 2020, and has confirmed that its next F1 challenger will be named 'C39'.
"We are also delighted to welcome Robert back home and we cannot wait to start working with him," team principal Frederic Vasseur said of Kubica.
"He is a driver that needs no introduction: one of the most brilliant in his generation and one who displayed the true meaning of human determination in his fight to return to racing after his rallying accident.
"His feedback will be invaluable as we continue to push our team towards the front of the grid."