0107 GMT November 21, 2019
In a contest which reflected Tunisia's shifting post-revolution political landscape, Saied, an independent, scooped almost 77 percent of the vote, compared to 23 percent for Karoui, Wataniya television said, according to AFP.
News of the victory triggered celebrations at the retired law professor's election campaign offices in central Tunis, as fireworks were set off outside and supporters honked car horns.
"Kais Saied, voice of the people," a gathered crowd chanted. "Long live Tunisia!"
"We are very happy. Tunisia has an honest man at the helm now. The difference between the two candidates was the work he has been doing," said Mustafa El Ghali, a family member.
The runoff was contested by two political newcomers – pitting Saied who is nicknamed "Robocop" against the businessman who is dubbed Tunisia's "Berlusconi".
They trounced the old guard in a September 15 first round, highlighting voter anger over a stagnant economy, joblessness and poor public services in the cradle of the Islamic Awakening.
Adding controversy and suspense to the contest, Karoui only walked free on Wednesday, having spent more than a month behind bars on suspicion of money-laundering.
The poll, Tunisia's second free presidential elections since its 2011 revolt, followed the death of former president Beji Caid Essebsi in July.
Saied, a 61-year-old constitutional law expert, called for Tunisians "to make a choice today in complete freedom".
"You have created a new concept of revolution, let your conscience guide you," he said, casting his ballot.
Saied campaigned upon the values of the 2011 revolution, based on opposition to Westernized and corrupt elites, and in favor of radical decentralization.