He took a further step toward relieving tensions with Russia by deciding to replace acting Foreign Minister Andriy Deshchytsya -- a hate figure in Moscow -- with his current envoy to ongoing OSCE-mediate negotiations with the Kremlin.
But the Western-backed leader also appealed for US and EU help to secure his crisis-torn country's porous border with Russia and stem the influx of arms and militants into the conflict zone.
"The peace plan begins with my order for a unilateral ceasefire," the Interfax-Ukraine news agency quoted Poroshenko as saying.
"Immediately after that, we must receive support for the presidential peace plan from all sides involved (in the conflict). This should happen very shortly."
Ukraine's acting Defence Minister Mykhailo Koval added that the ceasefire order would be issued "literally within days".
Poroshenko's comments followed a phone conversation with Russian President Vladimir Putin on Tuesday in which the two discussed a long-term solution to the pro-Kremlin uprising gripping Ukraine's eastern rustbelt.
The Ukrainian leader's office said the two presidents "discussed a series of priority measures that must be undertaken to implement a ceasefire, as well as the most efficient ways to monitor it".
The Kremlin confirmed that "the issue of a possible ceasefire in the area of the military operation in Ukraine's southeast had been touched upon."
Poroshenko's peace initiative calls for an end to hostilities and for Putin to formally recognise the new leadership in Ukraine that rose to power following the ouster of a pro-Russian administration in February after months of pro-EU protests.
The 48-year-old confectionery tycoon won Ukraine's May 25 presidential election on a promise to quickly end the country's worst crisis since its independence from the Soviet Union in 1991.
Interfax-Ukraine said Poroshenko told reporters in Kiev that the ceasefire was meant to be a temporary measure to give the pro-Russian militants a chance to disarm.
The rebels have previously rejected similar calls and vowed to continue a campaign to join Russia that has killed more than 325 civilians and fighters on both sides.
- Major irritant -
Poroshenko's decision to tap 46-year-old Pavlo Klimkin to replace Deshchytsya as foreign minister will help address one of the biggest irritants in relations with Moscow.
Klimkin is a veteran diplomat who recently served as Ukraine's ambassador to Germany and is now Poroshenko's personal representative at talks with Moscow that were launched on June 8 by the Vienna-based Organisation for Security and Cooperation in Europe (OSCE).
Deshchytsya became embroiled in controversy at the weekend when he called Putin "a prick" while trying to restrain protesters who attacked Moscow's embassy compound in Kiev.
Russian Foreign Minister Sergei Lavrov responded by saying that he had no intention of speaking to Deshchytsya again.
Ukraine's parliament is largely expected to approve Klimkin's candidacy in a vote later this week.
Poroshenko's talks with Putin and nomination as foreign minister of a figure who has already won a degree of trust from Moscow come in sharp contrast to the freeze in the two sides' relations that followed Russia's annexation of Crimea in march.
But Poroshenko has stressed that he will be unable to put an end to the fighting until Ukraine regains complete control of its 2,000-kilometre (1,230-mile) land border with Russia.
The US State Department on Tuesday once again expressed concern about "the movement of military tanks and other equipment across the border" into eastern Ukraine.
Poroshenko's office said he told German Chancellor Angela Merkel by telephone on Tuesday that Ukraine needed Western help sealing the frontier.
"Petro Poroshenko noted that EU and US assistance was essential for stepping up the state border's control," his office said in a statement.
The United States has pledged "non-lethal military aid" such as helmets and medical supplies for Ukraine's underfunded armed forced.
But Washington has refused to provide any combat equipment and rejects the idea of deploying ground forces in Ukraine.