Asked if he thought that the Syrian president was directly responsible for al-Hariri's killing in Beirut last February, Khaddam told Britain's Sky News on Thursday: "In my belief, yes, my personal belief is that he ordered it. But at the end of the day there is an investigation. They must give the final decision."
Khaddam, who moved to Paris after resigning in June, has said in interviews since December that Assad had threatened al-Hariri shortly before he was killed in a car bombing on 14 February. Al-Assad has denied the allegation.
In December, Khaddam told Al Arabiya television that the killing of al-Hariri could not have been carried out by Syrian agents without al-Assad's involvement.
Al-Assad is under intense international pressure to co-operate with a UN inquiry into the killing of al-Hariri. Syria said on Thursday it would not allow investigators to meet the president.
Khaddam has said he wants a popular uprising to overthrow the Syrian government, and Syrian officials call him a traitor. Asked if he would use the same word to describe al-Assad, he said: "Yes, I call him a traitor.
"Corruption in Syria is so widespread within the closed circle around the president. He's practised corruption so much that you see that his cousins control everything. And as far as foreign policy is concerned, his policy is causing humiliation to Syria."