Marquez began his first MotoGP campaign in April with a third-place finish in the season-opener at Qatar.
At the Circuit of the Americans race in Austin, Texas, he became the youngest rider in series history to win a pole and a race.
He failed to reach the podium only once this year, after a crash at a race in Mugello in June.
With his win on Sunday, he completed a sweep of the season's three US races. He also won at Laguna Seca last month.
"I didn't make such a good start," said Marquez, notching his third straight win and fourth of the season. He also won in Germany.
"But I knew before the race that the most important part was toward the end of the race. I just tried to follow Dani and Jorge. Then I tried to attack. Now we go back to Europe, and we would like to continue."
-AAP