A total of 39 former gendarmes stood trial before the 29th Istanbul High Criminal Court.
Among the accused was Gurcan Sercan, the commander of Istanbul's Provincial Gendarmerie Command at the time of the attempted coup.
Eleven of the convicts, including Sercan, got life sentences on charges of attempting to overthrow the constitutional order, said a judicial source, who asked not to be named due to restrictions on speaking to the media.
Twenty-eight of the convicts also received prison sentences on charges of assisting to overthrow the constitutional order and membership of a terror organization.
FETO and its U.S.-based leader Fetullah Gulen orchestrated the defeated coup of July 15, 2016, which left 251 people martyred and nearly 2,200 injured.
Turkey accuses FETO of being behind a long-running campaign to overthrow the state through the infiltration of Turkish institutions, particularly the military, police and judiciary.