Talks between the regime and Syrian opposition, due to resume next week in Geneva, aim to set up a political transition process to end the country's five-year-old war. A UN-brokered international roadmap foresees a transitional authority by the middle of this year and elections by mid-2017.
"Assad has to leave at the beginning of the process," the Saudi minister, whose country backs the Syrian opposition, told reporters in Paris. Referring to the sequence of events, he said: "There is a transitional body, power shifts from Assad to the transitional body, and then he goes."
"For us it is very clear, he leaves at the beginning of the process, not at the end."
Syria peace talks set for March 9 will begin the following day with participants due to arrive in Geneva over several days, UN envoy Staffan de Mistura said earlier Saturday.
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A first round of talks in early February was cut short amid intensifying Russian air strikes in Syria in support of Assad's forces.
The Saudi minister said there was no possibility that Assad could remain in power.
"The Syrian people have spoken when they took up arms against Bashar al-Assad and their message is very very clear: he is not going to be their president... They have already decided with their feet, with their guns," he said.
Discussing Syrian opposition reluctance to travel to Geneva to resume peace talks, he admitted that "they can't go into talks empty-handed".
"Our position is to support them 200 percent," said al-Jubeir. "We don't question. We may advise, express our views to them, but we support them," he added.