Poroshenko's decision, announced shortly after the much-violated 10-day cease-fire expired, raises the prospect of renewed escalation of a conflict that has killed more than 400 people.
A grave Poroshenko made a televised address early Tuesday vowing that "we will attack, and we will free our country." The cease-fire expired at 10 p.m. Monday.
There was no immediate sign of a response from Russia early Tuesday.
The idea behind the truce announced June 20 was to give pro-Russian rebels a chance to disarm and to start a broader peace process including an amnesty and new elections. Poroshenko, a wealthy candy magnate elected May 25, had already extended the cease-fire from seven days.
But rebels did not disarm, and the cease-fire was continually violated, with both sides blaming each other. Rebels called the cease-fire fake and did not yield to Poroshenko's latest push to get them to turn over key border crossings with Russia and permit international monitoring.
"The unique chance to put the peace plan into practice was not realized," Poroshenko said. "This happened because of the criminal actions of the fighters." He said the militants violated the truce "more than a hundred times."
Poroshenko said the government was ready to go back to the cease-fire "at any moment, when we see that all sides are keeping to the basic points of the peace plan."
Poroshenko said he made the decision after a meeting of the national security council. "After discussion of the situation, I, as commander in chief, took the decision not to continue the unilateral cease-fire."
"Ending the cease-fire, this is our answer to terrorists, armed insurgents and looters, to all who mock the peaceful population, who are paralyzing the economy of the region ... who are depriving people of a normal, peaceful life," Poroshenko said in his speech.
Poroshenko's decision followed four-way talks in search of a solution with Russian President Vladimir Putin, German Chancellor Angela Merkel and French President Francois Hollande on Monday as the deadline approached. He issued a statement after the talks ended, saying the key conditions needed to continue the cease-fire had not been met.
European leaders and the U.S. have urged Russia to use its influence with the rebels to ease the bloodshed and have threatened to impose another round of economic sanctions against Moscow.
While Putin has expressed support for the cease-fire, the West has accused Russia of sending weapons and fighters across the border into Ukraine. Russia says any Russians there have gone as private citizens.
Tension between Russia and Ukraine escalated in February when protests by people who wanted closer ties with the European Union drove pro-Russian president Viktor Yanukovych from office. Russia called that an illegal coup and seized Ukraine's Crimea region, saying it was protecting the rights of people there who speak Russian as their main language.
The insurrection in the eastern regions near the Russian border started soon afterward, with separatists occupying buildings and declaring independence.
Poroshenko said he meant for a cease-fire to be followed by an amnesty for fighters who had not considered serious crimes, and political concessions such as early local and regional elections, protections for speakers of Russian and, in the longer term, changes to the constitution to decentralize power to the regions.
The end of the cease-fire raises the question of what action the Ukrainian military can take. It has so far been unable to dislodge rebels occupying the city of Slovyansk or to retake control of three key border crossings with Russia. At one point, the rebels shot down a government military transport, killing 49 service members.