At 7.35 p.m. local time (1635GMT), 95.04 percent of votes had been counted.
Figures from vote counts showed Yes on 51.70 percent -- 23,997,153 votes -- while No had 48.30 percent, or 22,420,313 votes.
Turkey headed to the polls on Sunday in a referendum on a 18-article bill that would see the country switch from a parliamentary to a presidential system.
Anadolu Agency