The comedian had the support of almost three-quarters of the voters surveyed.
"I do not believe he will last long once he's elected", said Sergiy Fedorets, 62.
An emotional Poroshenko conceded defeat to his supporters, some of whom were crying.
"Zelensky doesn't have experience, and Putin is a very risky adversary", said Volodymyr Fesenko, a political analyst in Kiev. I am leaving the office, but I want to highlight that I am not leaving politics.
The poll conducted by the Kyiv International Institute of Sociology and the Razumkov Center public opinion organization was based on more than 13,000 responses to face-to-face questions at 300 polling places, two hours before the polls closed. "Well, at first it can be a bit amusing, and then it might hurt afterwards", he said after casting his ballot.
Poroshenko emphasized the need to "defend achievements of the past five years", noting the creation of a new Ukrainian Orthodox Church that is independent from Moscow's patriarchate, a schism he championed.
Zelenskiy has pledged to keep Ukraine on a pro-Western course, but has sounded less emphatic than Poroshenko about possible plans for the country of 42 million people to one day join the European Union and North Atlantic Treaty Organisation.
Harry Kane Could Play Again This Season - Pochettino
I honestly think he deserves it; he's so young. "If I lose it's because the opponent beat me or beat us", Guardiola said. But, that said, the fighting spirit and results that we have earned have been hugely enjoyable.
According to The Guardian, the poll suggested the Zelenskiy had won the presidential runoff with 73.2% of the vote against Poroshenko's 25.3%.
The comedian had been heavily favoured going into the election, holding a decisive lead in the polls in recent weeks.
Grigory Karasin, a Deputy Russian Foreign Minister, said the exit poll showed Ukrainians had voted for change, including in their foreign policy, the RIA news agency reported.
Mr Poroshenko during the campaign said that Mr Zelensky was too inexperienced a leader to be able to stand up to Russia's Vladimir Putin.
Poroshenko has sought to portray Zelenskiy as a buffoonish populist whose incompetence would leave Ukraine vulnerable to Russian Federation.
The stunning result for the 41-year-old, who secured almost three-quarters of the votes, is a bitter blow for incumbent Petro Poroshenko who tried to rally Ukrainians around the flag by casting himself as a bulwark against Russian aggression.
But Mr Zelensky fired back with well-rehearsed lines, including: "I'm the result of your mistakes and promises". "It's time to change that". However, the goals have been elusive amid Ukraine's economic problems, pervasive corruption and fighting in the east. O'Hagan says he inherited a hard situation in 2014 and implemented many reforms but has not convinced voters that he is serious about tackling corruption.
A comedian best known for playing the role of an accidental president on television easily won the real-life election for president in Ukraine on Sunday, exit polls indicated, putting a political neophyte at the helm of a country at the center of the West's geopolitical struggle with Moscow.
U.S. President Donald Trump phoned Zelenskiy and pledged to support Ukraine's territorial integrity, while European Council President Donald Tusk congratulated the Ukrainian people on what he called a show of democratic maturity.