Liberians are choosing a new president in a run-off vote between Vice-President Joseph Boakai and former international footballer George Weah.
Mr Weah, 51, won the first round, but did not secure the required 50% of the vote for an outright victory.
Legal challenges delayed the vote to replace Ellen Johnson Sirleaf, Africa’s first elected female president.
Liberia, which was founded by freed US slaves in the 19th Century, has not had a smooth transfer of power in 73 years.
Polls across Liberia opened at 08:00 GMT and will close at 18:00.
More than two million people are eligible to cast their ballots.