The One Direction heartthrob was singing along with his bandmates at the gig in Pittsburgh when he suddenly dashed behind some set pieces and hunched over to vomit, according to reports.
He was then seen clutching a towel and water bottle to help settle his stomach, while the rest of the boys continued to perform Rock Me. They covered for Harry during what should have been his solo segments so that he could wait until he felt better to return to the stage.
An onlooker posted on YouTube: "I was there! He got dehydrated and overheated and ended up throwing up backstage."
Harry, 19, soon returned and apologized for missing a bit of the gig.
But the star apparently headed back too soon, as he had to dash off stage again later as the group performed Teenage Dirtbag.
Harry hasn't commented on being ill, but there have been reports that it followed a night of partying.