Rachid El-Boukhari, 35, had denied intending to kill the preacher at the main Shiite mosque in the Belgian capital.
Boukhari, who describes himself as Sunni, had risked a life sentence for carrying out the attack on the Rida mosque just before evening prayers on March 12, 2012.
But earlier on Friday, he was cleared of a "terrorist offence" but found guilty of "arson causing death" under the "aggravating circumstances" that the attack was "based on religion."
Prosecutors had asked the court for a 30-year sentence.
"Yes, I knew that there was at least one or two people in the mosque when I set it on fire," the defendant had told the Brussels court during the trial.
"But I thought they would leave through the door. The problem is that one of these people shut the door and we all found ourselves shut inside the building," he said.
"I didn't think a man would die. I just wanted to wake up the Shiites," he said, denying he intended to kill.
Boukhari was arrested at the scene after he allegedly hurled abuse at Shiites, blaming them for the repression in Syria. (Maybe he has not seen yet how his fellow takfiri terrorists are killing innocent people in Syria!)
Armed with an axe and a knife, he poured petrol on the building and set fire to it.
The fire spread quickly through the mosque and the imam, Abdellah Dahdouh, a 47-year-old father of four, succumbed to the fumes after having first tried to put out the blaze.
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