World number one Sinner was drawn in the Ilie Nastase Group with Daniil Medvedev, Taylor Fritz and Alex de Minaur at the tournament in Turin, which starts on Sunday.
Reigning French Open and Wimbledon champion Alcaraz, ranked three in the world, was drawn in the John Newcombe Group with Alexander Zverev, Casper Ruud and Andrey Rublev.
The top two players in each group will qualify for the semi-finals.
Sinner will face tournament debutant De Minaur on Sunday night after 2020 champion Medvedev kicks off the singles against American Fritz.
Alcaraz will open his bid for a first Finals title on Monday, against Casper Ruud before Alexander Zverev faces Andrey Rublev.
Sinner, who lost last year's final to Novak Djokovic, tops the world rankings after a stunning season in which he claimed the Australian and US Opens -- his first Grand Slam victories -- and won five more ATP titles.
However he has been dogged by controversy after twice testing positive for traces of the steroid clostebol in March.
He was initially cleared by the International Tennis Integrity Agency but at the end of September the World Anti-Doping Agency appealed, seeking a ban of up to two years.
ATP Finals group stage draw:
Ilie Nastase Group: Jannik Sinner, Daniil Medvedev, Taylor Fritz, Alex de Minaur
John Newcombe Group: Alexander Zverev, Carlos Alcaraz, Casper Ruud, Andrey Rublev
