"The mujahedeen... Are defending the entire ummah (Islamic community). Trust them and give them moral, financial, military and political support," said a statement signed by 55 Saudi clerics speaking out against the "crusader alliance".
Russian warplanes have been conducting raids in Syria since last Wednesday.
Moscow insists it is attacking only Islamic State group militants.
But the United States Pentagon says Russian jets have targeted other rebel groups in order to support the regime of Syrian President Bashar al-Assad.
Gulf states also belong to a US-led military coalition bombing IS targets in Syria since last year.
Thousands of foreign fighters, including from Saudi Arabia and other Arab states, have joined the jihadist IS which has seized large parts of Syria and Iraq.
The group has carried out beheadings, crucifixions and other atrocities.
In their statement, the Saudi clerics accused Western nations of failing to protect the Syrian people.
They called on Saudi Arabia and other Sunni countries to guard Syria "from the influence of the Russians and the Persians", a reference to Riyadh's regional rival Shiite Iran.
