A total of 191,885 ballot boxes have been set up for voters in the country.
Voters will choose between President Recep Tayyip Erdoğan, who is seeking reelection, and main opposition candidate Kemal Kiliçdaroğlu.
Overseas voting was held until May 24. As for customs gates, voters will be able to cast their ballots until domestic polling stations close at 5 p.m. local time (1400GMT) on Sunday.
Millions of voters went to the polls on May 14 to elect the country's president and members of its 600-seat parliament.
Erdoğan's People's Alliance won a majority against the opposition's six-party Nation alliance in parliament.
But the presidential race will be decided in Sunday's runoff since no candidate could get the required 50% in the first round, though Erdogan was in the lead.
