Greek conservative leader Kyriakos Mitsotakis has thanked voters for giving his party a "strong mandate" to move faster on the path of change.
He beat his centre-left rival Syriza in May, but called new elections in a bid to win enough seats to govern alone.
His New Democracy party was polling 40.5%, 20 points ahead of Syriza, with more than 90% of the votes counted.
The vote came 11 days after a migrant boat tragedy off Greece in which 500 people are thought to have died.
Although three days of mourning were held, the disaster had little effect on the campaign and Greeks voted to maintain economic stability.
Under Greek election rules for a second election, the conservatives are awarded a bonus number of seats in parliament, but the exact number depends on the final result. With more than 40%, New Democracy could have a 50-seat bonus.
"The people have given us a secure majority," said Mr Mitsotakis as the result became clear. "Major reforms will go ahead with speed."
Last month, his party had fallen just short of a majority in the 300-seat parliament and he called a second election in a bid to form a stable, single-party government.
He said he could not promise miracles, but that New Democracy had big aims for a more efficient state, and a better health service and education.
Former Prime Minister Alexis Tsipras's Syriza party was unable to narrow the margin of last month's defeat.
But one of the big stories of the election was the success of a newly created far-right Spartans party, which was set to pass the 3% vote-share threshold to enter parliament.
The Spartans only emerged as a political force this month when the Greek Supreme Court banned another far-right party, the Greeks, and its jailed founder, Ilias Kasidiaris, threw his weight behind them.
Kasiadaris had been the spokesman for neo-Nazi party Golden Dawn, which was banned as a criminal organisation and its leaders given long prison terms.
Greek commentators noted that together with nationalist Greek Solution and ultra-conservative Niki (Victory), the hard right had won a bloc of more than 30 seats.
The victory secured by Kyriakos Mitsotakis, 55, over Alexis Tsipras's Syriza is a rarity in Greek politics, as few parties increase their share after a first term in office.
He is credited with successfully returning the Greek economy to stability and growth after a severe debt crisis and three international bailouts. Mr Tsipras had led Greece at the peak of the crisis from 2015.
Although many Greeks are struggling with the cost-of-living crisis, voters chose to stick with the party promising lower taxes and improved public health.
His party was helped by the fragmentation of the left-wing vote; with Socialist PASOK set for more than 11% and the Communist KKE on around 7%.
The conservative leader has formed a reputation as a Teflon-coated leader, fending off a series of damaging crises in the past year, including a rail disaster and a wire-tapping scandal that brought down the intelligence chief and his own nephew, who worked as the prime minister's chief of staff.
Greece was being led by a caretaker government when a migrant boat sank off the south-west coast in the early hours of 14 June.
Since the migrant crisis, the views of most Greek voters have shifted in favour of stricter, more conservative policies, says Panos Koliastasis, assistant professor of politics at the University of Peloponnese.
"The reason is rooted in the 2020 migration crisis on the Evros [river], when Turkey tried to push thousands of migrants into Greek territory and the Mitsotakis government acted swiftly. So the greater part of the public perceives the migration issue as an external threat to national sovereignty."
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