Two people on one helicopter - a Greek and a Danish citizen - died, while the crew on the other, one person from Greece and one from the United Kingdom, survived, the fire service said.
Investigations had started into what caused the collision, it added.
Destructive blazes in France and Spain had showed signs of easing over the weekend but several wildfires have broken out in Greece after a period of relative calm there.
The two helicopters collided in the area of Psatha in the Attica region, Greece's fire service said in a statement.
Video posted on the Forecast Weather Greece Facebook page appeared to show two helicopters closing to cross paths at low altitude, before the main rotor of the lower helicopter struck the underside of the other.
The lower one then crashed in flames.
The second helicopter then dumped its load of water and flew away.
Reuters has not verified the authenticity of the footage.
Investigations suggested the fire that started in the Greek region of Boeotia before spreading to Attica was caused by sparks from conductors on a private network transmitting electricity generated by wind turbines to the main power grid, the eKathimerini website reported.
Two Greek citizens were arrested on suspicion of negligence while another suspect remains at large, the report said.
Gale-force winds that had fuelled a fire around Porto Germeno on the Gulf of Corinth 60km northwest of Athens subsided but flames crossed a mountain to the south and reached the settlement of Veniza and a military firing range, igniting unexploded ordnance.
Firefighters focused their efforts on preventing fire from advancing towards the coastal town of Megara, which has a population of about 30,000.
"There are moments when the force of nature and the intensity of weather conditions exceed all human planning and every operational capability," Prime Minister Kyriakos Mitsotakis said in his regular Sunday message.
Greek authorities issued emergency alerts for people to leave several settlements in western Attica, including an industrial estate near Megara.
In the village of Agios Konstantinos, residents were doing their best to halt the spread of the blaze.
"The fire has been burning since last night, it is certain that they cannot have forces everywhere. We as residents are doing everything we can," said Tasos Tzempelikos, 61, who along with his son was pouring water on the fire.
Greek daily Protothema reported that a major effort was ongoing to prevent the fire from Veniza from reaching the nearby coastal town of Megara.
It said more than 10,000 hectares of land had likely been destroyed by the fire so far.
Overnight, several settlements on the Ionian island of Kefalonia were evacuated after a fire broke out in the Pastra area in the south.
In France, wildfires in the Gironde and Var departments showed signs of easing.
Authorities reported only minor flare-ups in the southwestern Gironde area and said the blaze in the southeastern Var department had not spread overnight.
In Monfort-Sur-Argents, in the region of Var, Georges Blanc, who fled from the fire, returned home and was busy putting out hot embers to prevent a flare-up while firefighters watered the wider area.
"With the combined efforts of the two Canadairs (firefighting planes) and the firefighters, I think you could say all the houses were saved ... And now, well, I'm coming back to check, because since I saw the flames, I thought the houses wouldn't have survived," he told Reuters.
In central Spain, wildfires that ripped through tens of thousands of hectares over the past week were mostly under control although firefighters were dealing with reignitions in some areas.
In the western province of Caceres a forest blaze erupted on Saturday, prompting an order for about 800 people to leave the area.
Those residents were allowed to return on Sunday but remained confined to their homes as the fire was still active.
