The C-12 Huron, a 10-seat turboprop plane, went down at 2:43 p.m. seven miles southwest of Camp Humphreys, a U.S. base, the military said.
The pilot and the co-pilot died, the U.S. Eighth Army said in a news release. Their names were withheld pending notification of their families.
South Korean police Lt. Sung Yol-gap said there were no casualties on the ground.
Speaking by telephone from Asan, a town near the crash site, Sung said the plane crashed in a rice paddy near a restaurant. It plowed through garlic patches into a storehouse, destroying a chicken coop, a tractor and other farming implements. The building caught fire.
Local TV footage showed the burnt and shattered wreckage of the plane.
The U.S. military news release quoted a local resident as saying that he saw the plane "on fire, maneuvering away from the populated area of the town" before the crash.
Some 37,000 U.S. troops are stationed in South Korea, a legacy of the 1950-53 Korean War.