The second round pits incumbent Andrzej Duda, an ally of the conservative government, against socially liberal Warsaw mayor Rafal Trzaskowski.
Mr Duda topped the first round of voting with a convincing lead but fell short of the 50% needed to win outright.
The BBC's Adam Easton, reporting from a polling station in Warsaw, says there is a steady stream of people arriving and putting on face masks to vote, but no sign of the long queues seen two weeks ago in the first round, which saw the highest turnout in 25 years.
President Duda is an ally of the governing nationalist Law and Justice (PiS) party and if he loses, the opposition could push through major changes in Polish politics
Speaking on the last day of campaigning on Friday, Mr Duda said he would continue strengthening the Polish state, "built on our inviolable tradition which is sacred to all of us and in which we have been brought up for generations"
He said Polish voters would never have another chance to change Poland's direction