The ballots cast on Sunday were yet to be counted by Ukraine's Central Election Commission, but the exit polls coincided with the official results after the first round of election held three weeks ago.
Although he said he accepted the loss, he said he would not leave politics and that Zelenskiy would face strong opposition, a reminder that Zelenskiy will have to work with a sometimes volatile parliament before it is re-elected in October.
"We have grown poor under Poroshenko and have to save to buy food and clothing", said 55-year-old sales clerk Irina Fakhova.
"My team and I are ready to support the president in everything that gets us close to the European Union and North Atlantic Treaty Organisation".
It is likely to reverberate in Russian Federation and elsewhere across the former Soviet Union, where few other countries can claim a democratic system that would allow a comedian to unseat the sitting president.
Zelensky, the star of the Servant of the People television sitcom, where he fights corruption as a teacher-turned-president, benefited from the Ukrainians' fatigue of mainstream politicians. Although he instituted some reforms, critics said he had not done almost enough to curb the country's endemic corruption. Many voters blame the incumbent for the failure to end the war in eastern Ukraine and for allowing corruption to fester at the highest levels of government. People migrate to Poland in droves. At a polling station set up at a Kiev public school, Inna Dzhurynska, 52, pointed at her traditional Ukrainian embroidered shirt when asked whom she was voting for.
"The new generation [Zelensky's team] will hopefully bring new ideas and strength to raise our country from its knees".
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"I do not believe he will last long once he's elected", said Sergiy Fedorets, 62.
"We would not advise him to talk to Putin alone", a Western diplomat in Kiev said. He seems like a decent person.
The poll was organized by Alexander Razumkov's Ukrainian Center for Economic and Political Studies, the Ilk Kucheriv Democratic Initiatives Fund and the Kiev International Institute of Sociology. "I would like all the people to live well, not only a small part [of the population]".
Polls gave Mr Poroshenko, who has been in power since 2014, 25 per cent of the vote.
Zelenskiy's unorthodox style and background in entertainment have drawn comparisons to Italy's Beppe Grillo or to US President Donald Trump.
"We need to see what the first decisions are, the first appointments. I hope this president will help".
"Zelensky doesn't have experience, and Putin is a very risky adversary", said Volodymyr Fesenko, a political analyst in Kiev.
From Ukrainian-speaking regions in the west of the country to Russian-speaking territories in the war-torn east, many voters said they yearned for change despite fearing uncertainty.