Initial reports indicate the personnel, including five crew members, were on board the CH-53E when it executed a hard landing, the U.S. Defense Department said.
All on board were transported to a hospital in Yongsan, South Korea, the military said. Several hours after the crash, 15 were released and six others were listed in stable condition. The extent of their injuries wasn't reported.
The crew is from the 31st Marine Expeditionary Unit stationed in Okinawa, Japan.
"A comprehensive investigation will take place to determine the facts and circumstances surrounding this incident," the Pentagon said in a release.
The report said the helicopter, described as a heavy lift aircraft capable of carrying dozens of troops, made a hard landing near Jipo-ri Range, about 55 miles north of Seoul.
Yonhap reported the helicopter caught fire after the U.S. military personnel escaped. Most of the aircraft was destroyed, military officials said.
It was conducting routine flight operations in support of exercise Ssangyong, a Korean Marine Exchange Program and part of the U.S.-South Korean Foal Eagle drills.