Ivan Heyn, the undersecretary of foreign trade, hanged himself at the Radisson Hotel downtown, a Uruguayan official told AFP.
The official said Argentine President Cristina Kirchner had been so upset over the news that she was being monitored by a medical team.
Security was beefed up after the incident at the hotel, where some of the leaders attending the summit are staying. Police investigators could be seen on the 10th floor of the building.
Argentina's embassy confirmed Heyn's death, and said in a statement that Uruguayan authorities were taking "all the necessary legal steps with regard to this tragic incident."
Heyn, 33, was an economist who belonged to the Peronist youth group La Campora that supports the governments of Kirchner and her late husband Nestor Kirchner.
A former leader of the Argentine University Federation, Heyn occupied other positions in the Economy Ministry during Cristina Kirchner's first term. He took up his new post just 10 days ago.