It was the third title he has won this year and the ninth of his career.
Alcaraz, 19, closed the final at the Rafa Nadal center court with a forehand winner for his fifth consecutive straight-sets win at the tournament this year.
The second-ranked Alcaraz had not successfully defended a title in his career before Sunday. Nadal was the last player to win consecutive titles in Barcelona with three straight from 2016 to 2018. Alcaraz will also try to defend his title at the upcoming Madrid Open.
He is now 23-2 for the year. His other titles this season came at Buenos Aires and Indian Wells.
Alcaraz was playing in his fourth final in five tournaments this year, winning it with 26 winners and seven unforced errors.
He and Tsitsipas exchanged breaks early in the first set but Alcaraz eventually took control to comfortably secure his fourth consecutive win over the fifth-ranked Greek. He had also beaten Tsitsipas in the Barcelona quarterfinals last year.
Tsitsipas, 24, was trying to win his first title in Barcelona after twice losing the final to Nadal in 2018 and 2021. He was looking for his first title of the year, and 10th of his career.