The failure is thought to have occurred shortly after 7am on Sunday, leaving officials scrambling to restore electricity.
“A massive failure...in the system left Argentina and Uruguay without power,” a spokesperson for energy company Edesur said in a statement.
The company has 2.5m customers, according to its website.
Edenor, Argentina’s largest energy company, also tweeted about the failure, as millions woke to the blackout.
“Due to a general failure in the interconnection system, Argentina and neighbouring countries are without electricity,” the energy distributor said. “Minutes after 7:00 a general failure in the Argentine Interconnection System (SADI) affected the entire country. While the causes are unknown at the moment, it is estimated that in three hours the electricity service could begin to be gradually re-established.”
EPEC, another Argentinian energy company, said the outage had affected most of the country, including Córdoba, Santa Fe and Buenos Aires.
UTE, Uruguay’s electricity body, also confirmed the account on Twitter.
“At 7.06 a defect in the Argentine network affected the interconnected system leaving the entire national territory without service,” the organisation said.
UTE said work was underway to restore power. The company added that electricity had been restored to some cities on Uruguay’s coast.
More than 48m people live across Argentina and Uruguay.
Users on social media, claiming to be resident in the countries, shared photographs of the blackout.
Gustavo Lopetegui, Argentina’s energy secretary, said the restoration of power could take several hours, according to Telefe Noticas, a prominent Argentinian news program.