"This person was on Ukrainian territory illegally and therefore, in compliance with all legal procedures, he was returned to the country from where he arrived," spokesman Oleh Slobodyan stated in a post on Facebook.
Saakashvili's lawyer Pavel Bogomazov earlier said his client was detained in Kiev by Ukraine's State Border Service on Monday.
His detention was confirmed by Ukraine's Ministry of Internal Affairs.
Later in the day, the Polish Border Guard said in a statement that Saakashvili had arrived in Warsaw.
"Saakashvili has been accepted in Poland ... The request made by the Ukrainian side has served as grounds for his acceptance ... The fact that he is the husband of an EU citizen has contributed to a positive decision on the Ukrainian side's request, while considering it," the statement said.
In September, Saakashvili, who was stripped of Ukrainian citizenship in July while away from the country, forced his entry to Ukraine from Poland, helped by a crowd of his supporters.
The detention and deportation from Ukraine following his extravagant behavior in December 2017, when Saakashvili appeared on his Kiev rooftop and threatened to jump from the eight-story building, claiming he was being persecuted by Ukrainian President Petro Poroshenko.
On January 27, a court of appeals in Kiev sentenced Saakashvili to nighttime house arrest, on charges of links to criminal groups, of organizing anti-government rallies and of allegedly receiving $500,000 from Russia to finance protest actions aimed at bringing about a regime change in Ukraine.
However, Saakashvili denies all his charges and calls them "politically motivated."