“My attitude to it is negative. The former president should not have renounced his Georgian citizenship. He had insulted his country and the institution of presidency,” Margvelashvili said.
He added that citizenship should be more important for a person than career, and any other behavior is "disgraceful."
Saakashvili served as Georgia's president from 2004 to 2013, his rule criticized as authoritarian. He was barred from running for president again in 2013, and left for the United States.
In 2014, a Georgian court charged him with abuse of power and misappropriation of public funds. Georgia has repeatedly asked Ukraine to extradite Saakashvili.
Since his ascent to power last June, Poroshenko has been rallying a team of foreigners around himself to assist him in governing Ukraine.
In December 2014, he admitted having granted Ukrainian citizenship to US, Georgian and Lithuanian nationals, who were handpicked for Cabinet jobs, including a Georgian minister of health, a Lithuanian economic development and trade minister and a US-born finance chief.