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.
"We learned a lot this year and I am confident we can make a big step forward next season.
I won’t be a rookie anymore so there won’t be any excuses for me: I want to be competitive right from the start of the championship and repay the confidence the team has shown in me."
After seeing Renault opt for Esteban Ocon and talks with Haas fall through, Alfa were Hulkenberg's last realistic hope of remaining on the grid next year, and a long history with team boss Fred Vasseur might have played into his favour.
However, Hulkenberg will likely need to get his racing fix elsewhere, and has been linked with a move to DTM, although he dismissed them on Twitter.
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