The 16-year-old Frenchman, who is part of the Sauber Junior Team driver development programme, edged Red Bull protege Dennis Hauger in a closely-fought title battle in German F4.
He had previously been a standout on the local and international karting scene, and dominated French F4's Junior class in 2018 during the series' first season under FIA F4 regulations.
Pourchaire completed a day with Carlin and two days with ART during F3 post-season testing in Valencia, and ended it with the seventh-fastest time.
"This is a new challenge for me and I’m grateful to ART Grand Prix for giving me the opportunity to race in Formula 3," Pourchaire said.
"Coming from German F4, this is a big step up, but after the tests in Valencia I feel ready to take it. We had all the possible conditions, dry, drying or wet and I felt good in all of them.
“ART Grand Prix gave me some leads to work on and I know where I can improve. I know I have a good margin of progression and that’s why my main aim will be to be better and better week in week out and to be ready to fight with the front runners by the final stages of the championship.
"I still have a lot to learn, but with a team such as ART Grand Prix, I think I can make it. I’m young and I want to take the time to do things right."
Pourchaire is the second signing unveiled by ART for 2020, as the team confirmed SMP Racing-backed Formula Renault Eurocup frontrunner Alexander Smolyar as part of its roster last week.
After years of sustained dominance in F3's predecessor GP3, ART finished only third in the teams' standings in the new-look F1 support series this year, getting its sole race win courtesy of Renault junior Christian Lundgaard.