This is Alcaraz's fourth consecutive victory over Sinner, snapping Sinner's 26-match winning streak, which dated back to when he last faced Alcaraz in October.
The win solidifies Alcaraz's position as the favourite for the upcoming French Open.
Sinner, who recently returned from a three-month doping ban, was aiming to become the first Italian man to win the Italian Open since 1976.
Alcaraz will regain the world No. 2 ranking after this victory, which came in his third clay-court final this season.