AVA- Zelensky, who has no previous experience in politics but played a fictional president in a popular TV show, has won 73% of the vote, the poll forecast.
This surpasses his rival, incumbent Petro Poroshenko, who won just 25% of the vote, according to the poll.
Zelensky and Poroshenko battled in a run-off vote to decide the country's next president on Sunday.
Poroshenko has said he has admitted defeat and would step down as president this month but told his supporters he would not leave 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.
Zelensky 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."
The 41-year-old comedian led the first round of voting three weeks ago which listed 39 candidates on the ballot paper.
Zelensky 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.
Zelensky 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.