The charismatic reformer took over after Georgia's 2003 Rose Revolution but then fell from grace and is now wanted by Georgian prosecutors on charges of abuse of office.
The charismatic reformer took over after Georgia's 2003 Rose Revolution but then fell from grace and is now wanted by Georgian prosecutors on charges of abuse of office during his nine-year rule that he insists are politically motivated.
In a Facebook post early on Thursday Saakashvili confirmed the decision, following an indirectly worded statement on Wednesday by Ukraine's migration service.
"Today I am being subjected to the same approaches that are used by Ukraine's prosecutors or bureaucrats against regular Ukrainians, whose rights are spat upon," Saakashvili said.
"Poroshenko decided to deprive me of my citizenship in an underhanded way, while I am out of the country!"
Ukraine's migration service said Wednesday that Georgia had provided "materials on Mikheil Saakashvili" to Ukrainian prosecutors that were then studied by the migration service and a presidential commission on questions of citizenship.
It said Poroshenko has the right to strip people of citizenship when they submit untrue information or fake documents.
Poroshenko's official website has not yet published any decree.
The statement came after Poroshenko met Georgian prime minister Giorgi Kvirikashvili in Tbilisi last week.
Saakashvili wrote on Facebook on Monday that he had information that Poroshenko had also visited billionaire ex-PM Bidzina Ivanishvili, who leads the ruling party behind the scenes.
"Their secret meeting lasted more than two hours and was almost entirely about me," Saakashvili wrote.
Saakashvili is now in the United States, and the decree means he will be unable to return to Georgia, an MP from Poroshenko's bloc, Sergiy Leshchenko, wrote on Facebook.
"Practically they are forcing Saakashvili to take refuge in America and forget about Ukrainian politics," Leshchenko wrote.
Known for sweeping pro-Western reforms in his native Georgia which he led from 2004 to 2013, Saakashvili was a strong supporter of Ukraine's Maidan uprising and as a reward Poroshenko appointed him as governor of Ukraine's Odessa region in 2015.
Saakashvili quit in November 2016, however, saying he was being held back in his efforts to fight corruption among high-ranking officials.
The fluent Ukrainian speaker had repeated run-ins with some members of Poroshenko's inner circle and was frequently accused of having outsized political ambitions of his own.