The 13 were part of a larger cell of 32 and were convicted of "following the Takfiri doctrine", a term usually used to refer to Al-Qaeda, the official SPA news agency said.
They were also convicted of charges including fighting abroad, supporting fighters financially, and helping "misled" people travel to conflict zones, SPA said.
Scores of Saudis are believed to be in the ranks of radical extremist groups in areas of unrest across the Middle East, including Syria.
Some of the defendants were also convicted of possessing arms and undergoing weapons training.
One was also found guilty of "exploiting religious seminars to spread his deviant thoughts among his pupils".