One of the 20, Abd Allah Matar al-Shimmari, was given an additional sentence of five years in jail for travelling to Iraq and forging a passport.
Another three defendants were fined the equivalent of $10,000 and a fourth about $3400 in connection with the case.
They were accused of involvement in "aggressive actions against a friendly nation, raising funds for Islamist militants engaged in fighting against US forces in Iraq and possessing illegal weapons."
The men were arrested in a police crackdown in July last year after Syria extradited four Kuwaitis, two of whom were arrested after returning from Iraq while the other two were arrested while trying to cross into the country.
Twenty-one of the convicts are Kuwaitis, one is a Saudi national and the remaining two are "bidoons", or stateless Arabs.