The controversial vote is the second election since the start of a decade-long conflict, that has killed more than 388,000 people and displaced half the pre-war population.
Huge election posters glorifying Assad have mushroomed across the two-thirds of the country under government rule.
With opponents abroad barred from running and no voting in the swathes of territory outside his control, Assad faces just two virtually unknown challengers.
The official news agency SANA declared voting had started as planned at 7:00 am (0400 GMT) and state television showed long queues forming in several parts of the country.
Syrians can cast their ballots in more than 12,000 polling centres, and results are expected to be announced by Friday evening, 48 hours after voting closes.
The election takes place amid the lowest levels of violence since 2011 — but with an economy in free-fall.
More than 80 percent of the population live below the poverty line, and the Syrian pound has plunged in value against the dollar, with inflation skyrocketing.
Assad's campaign slogan, "Hope through work", evokes the colossal reconstruction needed to rebuild the country, requiring billions in funding.
Assad, a 55-year-old ophthalmologist by training, was first elected by referendum in 2000 after the death of his father Hafez, who had ruled Syria for 30 years.
He faces former state minister Abdallah Salloum Abdallah and Mahmoud Merhi, a member of the so-called "tolerated opposition", long described by exiled opposition leaders as an extension of the regime.
Assad has refrained from holding campaign media events and interviews.
But he issued a general amnesty for thousands of prisoners earlier this month, on top of a series of decrees that aim to improve economic conditions.
Syrian Interior Minister Mohammad Khaled al-Rahmoun on Tuesday said that 18 million Syrians at home and abroad were officially eligible to vote.
But the actual number of voters will likely be smaller, with wide swathes of Syria outside Assad's control, and with many refugees excluded.
Last week, thousands of Syrian refugees and expatriates cast an early ballot in the embassies of their host countries.
But Syrians who fled the country illegally — and so who could not show an exit stamp in their passport — were barred from voting.
Several countries that oppose Assad blocked the vote altogether, including Turkey and Germany, which host large Syrian refugee populations.
The US and the European Union said Tuesday the elections was "neither free nor fair," and Syria's fragmented opposition has called the polls a "farce".
But they will watch powerless as Assad prepares to renew his grip on power.
In Syria's rebel-held northwest, home to three million people, opposition activists on Tuesday distributed mock campaign posters ridiculing Assad in protest.
Kurdish authorities in the northeast, who have carved out a semi-autonomous zone, said they are "not concerned" with the election.
In the last multi-candidate poll in 2014, Assad took 88 percent of the vote.
It was Syria's first election in nearly 50 years, with Assad and his father Hafez renewing their mandates in successive referendums.
It took place as the war raged, with the air force bombarding rebel areas in Aleppo and fierce fighting in Hama, Damascus, Idlib and Daraa.
This time around, the frontlines are relatively quiet.
"Assad is running the risk of being the only certainty in a country in ruins," said a European diplomat following Syrian affairs.
Credit belongs to : www.manilastandard.net