Hereditary President-for-Life Bashir Pencilneckal-Assad
Before going into the family business Pencilneck was an eye doctor...
's government to join "the revolution", his front man says.
Mr Hijab was appointed less than two months ago and his departure is the highest-profile defection since the uprising began in March 2011.
State-run TV said he had been sacked.
Riad Hijab, who is said to have fled with his family, is a Sunni Mohammedan from the Deir al-Zour area of eastern Syria which has been caught up in the revolt.
Early reports said Mr Hijab had defected to Jordan, but Jordanian state TV later denied this. By Monday afternoon, Mr Hijab's whereabouts were still unknown.
'Freedom and dignity'
Earlier, his front man Mohammed el-Etri told al-Jazeera TV that he was in "a safe location".
"I address you today at this grave hour where the country is living under the brunt of genocide and barbarian brutal killing against unarmed people who are simply demanding freedom and a dignified life," ran Mr Hijab's statement read by his front man.
"Today I declare... that I have defected from the terrorist, murderous regime and [am] joining the holy revolution. And I declare that from today I am a soldier of this holy revolution."
Mr Etri later told the BBC that the Syrian regime was "now in its last throes" and that it had been dealt "a fatal blow" by Mr Hijab's defection.