STORY: While opinion polls have placed the ruling conservative New Democracy party in the lead, a change to the country's electoral system means it is likely to fall short of an absolute majority.
Polling stations opened at 0700 local time (0400 GMT) and are due to close at 1600 GMT. Just under 10 million Greeks have the right to vote. An exit poll by six polling agencies will be released at 1900 GMT.
New Democracy, headed by Prime Minister Kyriakos Mitsotakis, is polling between 31-38%, followed by opposition leftist Syriza, trailing by 4-7 points. Pollsters say a party would need more than 45 percent to win outright.
An inconclusive vote will trigger days of consultations among political parties to hammer out common ground. Should no party win outright, Greek President Katerina Sakellaropoulou will give the top three parties a three-day mandate each to form an administration.
If they all fail, Sakellaropoulou will appoint a caretaker government that will lead the country to new elections roughly a month later.