Zelensky, a political newcomer who plays a school teacher that is elected president on a Ukrainian TV show, won 72.7 percent of the vote over Poroshenko's 27.2 percent to secure a landslide victory in the second and final round of voting.
The comedian and actor, who played his country's president on television, is now set to take on the job in real life after winning Ukraine's presidential runoff vote in a landslide, according to an exit poll released Monday with almost all the votes counted.
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.
Zelenskiy's promise to fight corruption has resonated with Ukrainians who are fed up with politics as usual in a country of 42 million people that remains one of Europe's poorest almost three decades after winning independence from the Soviet Union.
It is well-known comedian Volodymyr Zelensky, who, according to preliminary results, convincingly defeated Petro Poroshenko.
His rival, Mr Poroshenko, who has been in power since 2014, admitted defeat but has said he will not be leaving politics.
Despite attempts by opponents to portray him as being soft on Russia, Ukraine is expected to continue on a pro-western path with Volodymyr Zelensky as president, seeking strong ties with the United States, Britain and other European states.
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The third season of Zelensky's show, "Servant of the People", aired this northern spring and includes scenes of a prosperous, corruption-free Ukraine in the aftermath of the Zelensky character's presidency.
"I promise I won't mess up", Zelensky said.
Zelenskiy has pledged to keep Ukraine on a pro-Western course while sounding 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. "It's time to change that".
But Mr Poroshenko's message fell flat with many voters struggling to survive on meagre wages and pay soaring utility bills. "More than anything I would like for the structure of our country to change, the whole system", he said.
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.
Mr Poroshenko's five years in office saw the creation of a new Ukrainian Orthodox Church independent of Moscow's church, a schism he championed.
"Ukraine has passed the democracy test", said Mykola, a 66-year-old pensioner. Although he instituted some reforms, critics said he had not done almost enough to curb the country's endemic corruption.
Germany has been a key broker in the conflict between Kiev and Russia-backed separatists in eastern Ukraine since Moscow annexed the Crimean peninsula in 2014, negotiating with France the now moribund Minsk peace accords. Zelenskiy, however, has largely stayed away from the campaign trail and eschewed interviews.