More than 3.5 million people are eligible to vote in the election, and both candidates - Georgian Dream-backed Salome Zurabishvili and opposition Grigol Vashadze - have promised to lead Georgia closer to membership in the European Union and NATO.
Georgia on Wednesday holds the second round of a knife-edge presidential election seen as a crucial test for the increasingly unpopular ruling party led by a billionaire oligarch.
The new head of state will be a largely ceremonial figure, but the vote is seen as a trial run for the contest between businessman Bidzina Ivanishvili's ruling Georgian Dream party and the opposition in more important parliamentary polls set for 2020.
Tensions have been high ahead of the vote, with some warning of potential unrest in the pro-Western republic, which has been shaken by civil wars and a 2008 conflict with Russia since gaining its independence from the Soviet Union in 1991.
In the first round held on October 28, the Georgian Dream-backed candidate, former French ambassador Salome Zurabishvili, failed to take the 50-percent-plus-one-vote needed to win outright.
Zurabishvili, 66, took 39 percent of the vote against 38 percent for 60-year-old opposition leader Grigol Vashadze, who is supported by exiled former president Mikheil Saakashvili's United National Movement (UNM) and 10 other groups.
Vashadze was narrowly leading in opinion polls ahead of the second round, and was given a major boost with the endorsement of ex-parliament speaker David Bakradze of the European Georgia party. He came third in the first round with nearly 11 percent.
A win for Vashadze would drastically change the country's political landscape, signalling a likely end to Georgian Dream's six-year dominance.
In a televised statement aired after the first round, Ivanishvili addressed voters, saying he heard their "heartache".
"I hear that you are dissatisfied, since your life did not improve," he said, looking pale and almost tearful.
In what critics derided as "vote-buying," Ivanishvili promised the government would drastically increase social spending and pledged to spend his own money to write off the bank loans of more than 600,000 individuals.
Tensions have risen after the opposition accused the government of voter intimidation and claimed that ruling party activists had attacked members of Vashadze's campaign staff.
Zurabishvili - who kept a low profile after the first round - said that she and her children received death threats through text and voice messages from people affiliated with the UNM.
In a series of coordinated statements, several senior Georgian Dream lawmakers have even raised the spectre of "civil war" if Vashadze is elected.
Vashadze, a career diplomat, has criticised Ivanishvili's "informal oligarch rule" amid growing discontent over the government's failure to tackle poverty.
Georgia's richest man, Ivanishvili stepped down as premier in 2013 after a year in office but is still seen as the country's de facto ruler.
The vote is Georgia's last direct leadership poll as it transitions to a parliamentary form of governance following a controversial constitutional reform.
The inauguration of a new president will pave the way for the new constitution to come into force, making the head of state a largely ceremonial figure.
The constitutional change was adopted in September 2017, despite opposition party protests.
The outgoing president, Giorgi Margvelashvili, refused to run for a second term, saying he was not interested in the reduced role.
Both Vashadze and Zurabishvili have promised to lead Georgia closer to membership in the European Union and NATO.
More than 3.5 million people are eligible to vote in the election, which will be monitored by international observers from the Organisation for Security and Cooperation in Europe.
Polls open at 0400 GMT and close at 1600 GMT, with results expected to start being released overnight.