Thessaloniki traffic police chief Giorgos Thanoglou says "millions" of the amphibians covered the tarmac Wednesday near the town of Langadas, some 12 miles east of Thessaloniki.
"There was a carpet of frogs," he said.
Authorities closed the highway after three car drivers skidded off the road trying to dodge the frogs. No human injuries were reported.
Thanoglou said the amphibians probably left a nearby lake to look for food.