Erdogan, 69, defied opinion polls and came out comfortably ahead with an almost five-point lead over his rival Kemal Kilicdaroglu in the first round on May 14. But he fell just short of the 50% needed to avoid a runoff.
Voting began at 8 a.m. (0500 GMT) and will finish at 5 p.m. (1400 GMT), Reuters reported.
The outcome was expected to start becoming clear by early evening.