Tyler Armstrong - of Yorba Linda in Southern California in the US - reached the summit on Christmas Eve with his father Kevin and a Tibetan sherpa, Lhawang Dhondup, his team says.
When they climbed back down to the base camp, Tyler was much less tired than his father and their guide, said Nicolas Garcia, who handled the expedition's logistics from the city of Mendoza.
"It's a record. Never before has a child as young as nine reached the summit of Aconcagua," he said.
There was a younger boy who climbed the lower slopes of Aconcagua, Mr Garcia added. He said an Inca boy was sacrificed around 500 years ago at 16,400 feet on Piramide, one of the mountain's lower peaks. Scientific tests on the mummy, recovered in 1985, put his age at about seven.
Only 30% of the 7,000 people who obtain permits to climb Aconcagua each year make the summit, Mr Garcia said, and no one under 14 is usually allowed to attempt it.
The Armstrongs hired a lawyer to argue before an Argentine judge that Tyler could safely accomplish the feat. He had already climbed the 19,341-foot Mount Kilimanjaro in Tanzania at the age of eight.
With Aconcagua conquered, he is determined to reach all "seven summits", the highest mountains on each of the seven continents.
Most of the tallest mountains in the world are in the Himalaya and Karakoram ranges to the south and west of the Tibetan plateau in the Northern Hemisphere.
The world's tallest mountain, Mount Everest, sits at 29,029 feet, while the second tallest, K2 is 28,251 feet.
Last year, three climbers died on Aconcagua, although no one died the year before that and no one has been killed so far this summer climbing season, Mr Garcia said.
Since the first climbers reached the top of Aconcagua in 1897, more than 110 people have died trying, he said.
"Tyler is a really happy kid, very open. And he's prepared for these climbs very carefully," Mr Garcia said.
"Neither he nor his father are in for 'adventure.' Their project is pretty conservative, with a guide who is very experienced, so from my perspective, their climb wasn't imprudent."
Describing the summit, Tyler said: "You can really see the world's atmosphere up there. All the clouds are under you, and it's really cold. It doesn't look anything like a kid's drawing of a mountain. It's probably as big as a house at the summit, and then it's a sheer drop."
"Any kid can really do this, all they have to do is try. And set their mind to the goal," said Tyler, who exercised twice a day for a year and a half to prepare for the climb.
He also held fundraisers, not only to cover the trip but to raise money for CureDuchenne, which funds muscular dystrophy research.
"I think Tyler's record speaks for itself and because I think he's doing it for a good cause, he's doing it to help other people, I think the judge recognised that," said his father, an emergency medical technician.
Mother Priscilla Armstrong is a paediatric neuropsychologist and they have another son, Tyler's younger brother Dylan.
"Most people think we as parents are pushing Tyler to do this, when it's completely the opposite. I wouldn't climb it if I didn't have to, but my wife makes me do it to keep watch on him," Mr Armstrong said.
"He's a great dad," Tyler said. "At 20,000 feet, he wanted to turn around but I kept him going. And the day we were getting off the mountain, he had a blister and it popped ... He made it to the summit and everything, but that dang blister made him ride a mule."
The previous record-holder for Aconcagua was Matthew Moniz of Boulder, Colorado, who was 10 when he reached the summit in 2008.
Mrs Armstrong, back home in Southern California, was apparently worried about her husband and son.
"Didn't sleep a wink thinking about you honey," she posted on his Facebook page while they were climbing down from the summit.
"Praying for you guys! Good luck and can't wait to hear from you!"