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.
A road official at the scene of the invasion told Alpha TV the situation was "crazy."
"About an hour ago when the tarmac heated up, they went away. When it cools, they'll be back up here," he added.