Poroshenko, 48, confirmed his candidacy late on Friday. Several opinions polls already had him in the lead even before he said he would run to succeed ousted president Viktor Yanukovich.
(Read more: Vitali Klitschko: Putin worried over Ukraine)
Poroshenko was an early and influential supporter of the popular uprising that toppled Yanukovich in late February, three months after he spurned a deal on closer ties with the European Union and plunged the country of 46 million people into turmoil.
Ukrainians will vote under the shadow of Russia's annexation of Ukraine's Black Sea Crimea peninsula.
Like Tymoshenko, who announced her candidacy on Thursday, Poroshenko promised to strengthen Ukraine's armed forces and protect its borders, which the West fears are still under threat from a possible Russian incursion into Russian-speaking regions of eastern Ukraine.
"We need to build a new, efficient and modern Ukrainian army, which will defend the sovereignty and integrity of our country," Interfax news agency quoted Poroshenko as saying in the town of Vinnitsa southwest of Kiev.
He officially submitted his candidacy to the election authorities in Kiev on Saturday.
Klitschko, addressing a meeting of his UDAR (Punch) party, urged supporters to back Poroshenko, and announced he would run instead for mayor of the capital.
"The only chance of winning is to nominate one candidate from the democratic forces," Klitschko said.