"God willing, the elections in Syria will be carried out without a hitch," said Ali Akbar Velayati, the senior foreign policy adviser to Iran's supreme leader, Ayatollah Ali Khamenei.
"This election will strengthen the legitimacy of the Bashar government... as his people have realised he has prevented Syria from disintegrating or falling to occupation," Velayati told the official IRNA news agency.
Iran has been a staunch ally of the Syrian leader throughout the uprising against his rule that erupted in March 2011.
It has provided both financial and military support, although it has repeatedly denied rebel claims that it has sent combat troops.
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Assad is expected to win comfortably in Tuesday's election, in which he faces two little-known challengers.
Voting will be held only in government-controlled areas inside Syria, prompting accusations from the armed opposition and its Western backers that the election is a mockery of democracy.
Some 200,000 people already voted at Syrian embassies around the world on Wednesday, officials in Damascus said, a small proportion of a diaspora population now estimated at some three million following a huge exodus of refugees.
The conflict has killed more than 160,000 people and prompted nearly half of Syria's population to flee their homes.