Although the post is largely ceremonial, the president has make-or-break power over the nation's fragile ruling alliance and the power to dissolve parliament in the event of a crisis.
Since inconclusive elections in October, Portugal's minority Socialist government has been relying on a delicate coalition with the extreme-left to run the country of 10.4 million people.
The overwhelming favourite for the next head of state is a TV pundit, Marcelo Rebelo de Sousa.
"I voted for professor Marcelo. I have been seeing him on television for years and I know his political beliefs," said Mario Machado, a 72-year-old pensioner, speaking in a posh Lisbon quarter.
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The 67-year-old law professor has been involved in Portuguese politics and media since his youth, co-founding a weekly newspaper in his 20s and helping to establish the centre-right Social Democratic Party.
Starting in the early 2000s he made his debut as a political analyst on TV, delivering clever commentary to a viewership that quickly grew.
His popularity is widely expected to help him break the 50-per cent mark for an outright win in today's voting. If none of the 10 candidates breaches this threshold, a run-off will be held on February 14.
The centre-right bloc of former conservative prime minister Pedro Passos Coelho won the most seats in the October ballot, but lost the absolute majority it had enjoyed since 2011. The government of Socialist Prime Minister Antonio Costa has promised to implement a moderate programme that upholds European Union budget commitments.
Rebelo de Sousa is a long-time conservative who has the backing of right wing parties but who claims total independence.