Petro Poroshenko said many current parliamentarians were backers of ousted President Viktor Yanukovych and that the majority of Ukrainians wanted a new parliament.
Elections would be held on October 26, he said in a TV address.
Separately, Ukraine’s military says it clashed with rebel armored vehicles that entered the country from Russia.
It said a column of 10 tanks and two armored personnel carriers was heading towards the south-eastern port of Mariupol but was stopped. Two tanks were reportedly destroyed.
More than 2,000 people have died in months of fighting between Ukrainian forces and separatists in Donetsk and Luhansk regions.
The two regions declared independence from Kiev, following Russia’s annexation of the southern Crimean peninsula from Ukraine in March.
Ukraine accuses Russia of arming the rebels in the east and sending its troops into the country – a charge the Kremlin denies.
On Tuesday, Petro Poroshenko is expected to meet his Russian counterpart Vladimir Putin at talks in Minsk Belarus.
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