It is the second grand slam final Ruud has lost this year, after the loss in the French Open.
Even then, a Spaniard stood for the opposition, when Rafael Nadal took home the championship in straight sets.
Forced a tiebreak
The US Open final was a much more even story.
The combatants started by winning each set, and in the third it was Ruud's advantage.
But after losing the set point, Alcaraz managed to force a tiebreak, which he then comfortably took home.
It turned out to be decisive, and in the fourth set the Norwegian did not have much to counter.
Gets $2.6 million
The championship ended with a serve, then Alcaraz lay on his back on the court, and hid his face in his hands, before there was a hug party in the stands.
Carlos Alcaraz thus writes history by becoming the youngest tennis player on the men's side to become world number one at the age of 19.
He is, however, a bit short of the record holder Martina Hingis, who became world number one as a 16-year-old in 1997.
In addition to the honor, the winner gets $2.6 million, twice as much as the final opponent.