The court in the western city of Innsbruck announced its ruling late on Wednesday after a jury found the 27-year-old guilty of 20 charges of "murder as a terror offence".
The man, who denied the charges, was detained in a refugee shelter in Tyrol state last June after being denounced by a fellow Syrian.
According to Austrian media, the accused had boasted to other asylum seekers of belonging to a rebel unit and shooting unarmed or injured troops loyal to Syrian President Bashar-al Assad.
After his arrest, the man initially admitted to the 2013-2014 killings in the western region of Homs but later retracted his confession, saying his testimony had been badly translated.
The translator rejected this in court: "The defendant told me he had shot badly wounded soldiers. I asked him to repeat his claim and he did."
The man, who was not named, was tried for war crimes under Austrian law.
Austria could not extradite him to face charges in Syria because of the bloody six-year civil war pitting Assad's troops against rebel groups.