"Bashar al-Assad is the Islamic Republic of Iran's red line because he was elected president by the Syrian people," Velayati said, according to IRNA news agency.
The fate of President Assad has been a stumbling block in the Syrian peace talks, with Washington and its allies insisting he must step down, and Moscow saying the people of Syria should decide on his future.
The Iranian politician also said that only the Syrian people must decide the future of their country without foreign interference.
Talking about the relations between Russia and Turkey, the advisor said that Tehran is obliged to ease tensions between the two governments.
"There is no benefit to tensions mounting up in the region. We must not take the side of either party, and have a duty to reduce tensions between these two countries," Velayati said.