The government declared three days of national mourning after what it called "an unprecedented catastrophe."
Hardest hit was the city of La Plata, where officials said 51 people died, the BBC reported Friday.
Six people died in the capital of Buenos Aires, officials said.
Another 20 people were reported missing.
La Plata Mayor Pablo Bruera said the flooding destroyed about 100,000 houses.
Provincial officials said 16 inches of rain fell on La Plata in a 2-hour span Tuesday. The storm dumped nearly 6 inches of rain on Buenos Aires.
Rains had stopped and floodwaters receded but electricity hadn't been fully restored, the BBC said. Two of La Plata's hospitals were affected by the power outage.
The government said it was distributing water, food and clothing at dozens of shelters across the province.
"We've never seen anything like it," provincial Gov. Daniel Scioli said. "People were taken by surprise, and some didn't have time to escape this deadly trap."