The Spanish teenager beat Casper Ruud 6-4 2-6 7-6 (7-1) 6-3.
Newsonline report that both Alcaraz and Ruud went into the match hoping to lift a first Grand Slam title and to top the rankings.
But it was the 19-year-old who came out on top on Arthur Ashe Stadium.
Alcaraz is more than a year younger than Lleyton Hewitt was when he topped the standings in 2001.
“This is something I dreamed of since I was a young kid. To be number one in the world, to be the champion of a Grand Slam.
READ ALSO Iker Casillas Names Current Best Goalkeepers In The World, Lists Top Five
“All the hard work I did with my team, my family. I’m just 19 years old so a lot of the decisions are with my team, my parents. It’s something that is very special for me,” Alcaraz said.
Alcaraz admitted he was “a little bit” tired, but said: “I always said it’s not time to be tired in the final round of a Grand Slam. You have to give everything on court, everything you have inside.”
