At least 24 students were reportedly left unconscious after freshmen packed their pillow cases with helmets and body armour before the brawl at West Point in New York.
Photos and video of plebes, as freshmen are called, with bloody faces and bloody pillows were circulated on social media.
“Four concussions, 1 broken leg, 2 broken arms, 1 dislocated shoulder, and several broken ribs. That’s one hell of a pillow fight,” one cadet posted on Twitter.
The fight took place on 20 August, but The New York Times said West Point did not confirm the events until Thursday.
Lt Col Christopher Kasker told the newspaper the annual fight is organised by first-year students as a way of building camaraderie after a tough summer of training.
He said upperclassmen overseeing the fight required cadets to wear helmets, but video shows many did not.
The injured cadets have returned to duty, and none of them have been disciplined, he added.
“West Point applauds the cadets’ desire to build esprit and regrets the injuries to our cadets,” he said.
“We are conducting appropriate investigations into the causes of the injuries.”
The publicly-funded academy is where many of the US Army’s top leaders are trained. It has held the pillow fights since at least 1897.