"I pledge to protect the sovereignty and independence of Ukraine," the newly inaugurated president said during the ceremony in parliament in Kiev.
Zelensky also said that his first task is to end the conflict with Moscow-backed separatists in the country's east.
"Our first task is a ceasefire in the Donbass," the newly inaugurated president said during a ceremony in parliament in Kiev, referring to the eastern region.
Zelensky announced the dissolution of parliament in his inaugural speech in order to hold early elections.
"I dissolve the Verkhovna Rada of the eighth convocation," the newly sworn-in president said during the ceremony in parliament in Kiev.
The next parliamentary polls had been scheduled for October.
Comedian Volodymyr Zelensky won a landslide victory in Ukraine's presidential election on Sunday. He told his supporters he would not let them down and encouraged other-post Soviet countries to bring new leaders to power.
The 41-year-old has been elected president of Ukraine twice -- first as a character in the popular TV show "Servant of the People", now for real.