Television comedian Volodymyr Zelenskiy, who has no political experience and few detailed policies, beat the incumbent Petro Poroshenko who has been in power for five years.
Volodymyr Zelenskiy, a comedian who played a fictional president on a popular TV show, will become Ukraine's next president after winning the election by a landslide.
Official results show the entertainer, who has no real previous experience in politics, has won 73% of the vote - with 95% of votes counted, according to the central election commission.
This surpasses his rival, incumbent Petro Poroshenko, who got just 25% of the vote.
Declaring victory at his campaign headquarters to emotional supporters on Sunday night, Zelenskiy promised he would not let the Ukrainian people down.
“I’m not yet officially the president, but as a citizen of Ukraine, I can say to all countries in the post-Soviet Union look at us. Anything is possible!”
Zelenskiy and Poroshenko battled in a run-off vote to decide the country's next president on Sunday.
Accepting defeat, Poroshenko hailed the election as a victory for democracy in Ukraine. He told his supporters he would not be leaving politics.
"Dear Ukrainian, this month I will leave the post of the head of state. This is how the majority of Ukrainians have decided and I accept this decision. I will leave the office but I want to announce firmly: I will not leave politics, " he said.
Zelenskiy told a crowd of supporters on Sunday "we did it together. Thanks to everyone. Now there will be no pathetic speeches, I just want to say - thank you."
Euronews was at Zelenskiy's campaign headquarters earlier on Sunday. See the atmosphere in the video below.
The 41-year-old comedian led the first round of voting three weeks ago which listed 39 candidates on the ballot paper.
Zelenskiy and Poroshenko met in an unconventional debate at a football stadium in Kyiv on Friday after the comedian challenged the incumbent president to a debate on social media. Both have pledged to keep Ukraine on a pro-Western course.
The actor is best known for playing an ordinary teacher who is unexpectedly elected to the presidency after an angry rant about corruption was posted online by his students.
However, Poroshenko, who has been the leader since 2014, has warned his opponent's lack of experience would leave Ukraine vulnerable to Russia.
The country has been fighting a war against Russian-backed forces in the east and whoever is confirmed as the next president will also inherit the West's standoff with Russia following Moscow's annexation of Crimea.
Zelenskiy has proved popular with his campaign to end corruption, ramp up living standards, and end the war in the eastern Donbass region but he has not overtly specified how he intends to do so in the election campaign.
However, on Sunday he said he planned to continue European-backed talks with Russia on a largely unimplemented peace deal and would try to free Ukrainians imprisoned in Russia, which is holding 24 Ukrainian sailors among others
European leaders Emmanuel Macron and British foreign minister Jeremy Hunt congratulated the comedian. European Council President Donald Tusk also welcomed the apparent victory in a tweet.