Dozens more were injured when strong winds hit the region of Halkidiki, near the city of Thessaloniki, authorities added.
"Six tourists were killed and at least 30 people were injured during this cyclone".
Two people were killed by falling trees and another two, said to be a Czech couple, died when gale-force winds overturned the vehicle they were in, which is believed to have been a caravan.
Elsewhere in the region, a woman and an eight-year-old boy, both Romanian nationals, were killed after a roof collapsed on a restaurant in Nea Plagia.
Powerful storms raged over Greece's northern Halkidiki region Wednesday, leaving at least six people dead and 20 injured as trees and electricity pylons toppled over, power was knocked out, and roads were blocked.
Two Russian nationals, a man and young boy, were also killed after they were caught under a collapsed tree.
Streets in towns in the area were dotted with uprooted pine trees and overturned motorcycles, local media reported.
According to port police, a fisherman in his sixties was also missing.
Greek Citizen Protection Minister Michalis Chrysochoidis expressed his sorrow for the victims and dos condolences to their families, saying crews are working around the clock to restore damages and electricity in the affected areas.
Such severe storms are unusual in Greece, where summers are typically hot and dry.
Athansios Kaltsas, the director of the Nea Moudania Medical Centre, told Greek television that it was "the first time in my 25-year career that I have lived through something like this".
'It was so abrupt, and so sudden, ' he said.
A state of emergency was put in place on the Halkidiki peninsula amid what has been described by meteorologists as "extremely unusual" conditions for the time of year.
Meteorologists forecast it would continue to rain in the area until about 9 a.m. on Thursday.
The storms came after temperatures in Greece soared to 37 degrees Celcius over the past two days.