The industrial towns of Mandra, Nea Peramos and Megara, west of the capital Athens, were the most affected.
Many of the dead were elderly people whose bodies were found inside their homes, reports say. Fast-flowing torrents of red mud flooded roads.
Prime Minister Alexis Tsipras declared a period of national mourning in the wake of the tragedy.
“Everything is lost. The disaster is biblical,” Mandra Mayor Yianna Krikouki told state broadcaster ERT.
At least 37 people have been taken to hospital, the broadcaster said, and some are still missing.
Bad weather has hit parts of Greece for about a week, but particularly heavy rain overnight caused the sudden flooding, for which locals said they were unprepared.
The force of the water moved vehicles, damaged walls and roofing, and left many homeless as their homes flooded to a life-threatening level.
By Wednesday afternoon, Greece’s fire service said it had received over 600 calls for help and dispatched almost 200 firefighters in 55 vehicles to the towns, which have a combined population in the tens of thousands.
“The water came down the mountain, millions of tonnes,” Stavros Fotiou, the deputy mayor of Nea Peramos, told ERT.
“Our roads are completely destroyed… 1,000 homes have been flooded, that’s a third of the town,” he added.
The region’s deputy governor, Yiannis Vassileiou, told the broadcaster that emergency services had been prepared for poor weather, but then “the Niagara Falls came down and could not be stopped”.
Prime Minister Tsipras said that declaring a period of national mourning was “the least we can do”.
He also vowed to provide aid to the victims and ensure they were housed safely. (Courtesy BBC)