ATHENS—Greece elected a conservative government on a promise to cut taxes and spur economic growth, initial results showed, ejecting a left-wing government that couldn’t deliver an end to the country’s long economic slump.

The center-right New Democracy party was set to win an absolute majority in Parliament in Sunday’s vote, making its pro-business leader, Kyriakos Mitsotakis, the next prime minister. With nearly all the votes counted, the conservatives were set to win 39.8% of the vote.

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