News of the Italian's contract extension means Nico Hulkenberg will likely depart F1 in 2020.
Giovinazzi has scored points in Austria, Italy and Singapore this season, but trails vastly experienced team-mate Kimi Raikkonen by 27 points in the drivers' championship, having contributed just four to Alfa's total in 2019.
However, the Ferrari academy driver had out-qualified Raikkonen in four straight races before the United States Grand Prix, with strong performances masked by Alfa's struggle to develop the car in 2019.
Giovinazzi said: "I am grateful for the incredible level of support I received in my first full season in Formula One: everyone in the team has stuck by my side since I joined and I am really looking forward to continuing our journey together.
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