Alcaraz, 19, beat Ruud 6-4, 2-6, 7-6 (1), 6-3 at the US Open in Flushing, New York, on Sunday.
He thus became the first male teenager to win a Grand Slam title since Rafael Nadal won the French Open in 2005. Alcaraz will now be the youngest tennis player to be the world number one.
On Saturday, tennis player Iga Swiatek of Poland, the world’s number one, won against Ons Jabeur of Tunisia in the US Open women’s singles final.
Swiatek, 21, secured a 6-2, 7-6 (7-5) win over Jabeur, 28, who is ranked fifth in the world.