The 23-year-old Spaniard took the ninth Masters 1000 title of his career and has now won seven of the nine events at that level. Serving at 1-2, Alcaraz faced four break points but held when Shelton misfired at the end of a seven-minute game. Alcaraz took the tie-break by winning four of the last five points from 4-3 down.
In the two months since New York he has won three titles and lost once in 17 matches. He will head into his next event on a nine-match winning streak, after titles in Vienna and Paris.
Shelton broke for the first time in an error-strewn third service game of the second set. Two games later he was two break points down, answered with a drop volley and a passing shot to reach deuce, and held.
He did it against the man who ended his U.S. Open title defence two months earlier. Their quarter-final in New York had gone to five sets; Paris did not.
He saved all six break points against him in the opening set. Shelton also stayed composed, seeing off two break points of his own.
A Shelton error at 4-4 and a leaping backhand smash gave Alcaraz two set points. The first went when his shot hit the top of the net cord and dropped back on his side of the court. Shelton then overcooked a forehand, and the set was gone.
He had said, after reaching the final, that his reflexes would need to be"ten out of ten"against Shelton's serve. Three games later they were, as he levelled the first set at 4-4."Today, it was one of the toughest challenges we have in tennis right now, facing Ben Shelton,"Alcaraz said after the match.