Joe Robinson, 25, was arrested on holiday accused of fighting the Islamists alongside the People's Protection Units of Syrian Kurdistan (YPG), which the Turkish state is hostile towards.
Robinson, formerly of Accrington, Lancashire, was sentenced to seven years and six months in prison, but he reportedly remains on bail while planning an appeal.
He has previously admitted fighting IS but went on to deny he acted alongside the Kurdish militia, considered by Turkey as a terror organisation because of its links to the Kurdistan Workers' Party which is fighting for autonomy for the region.
Robinson instead insisted he travelled to Syria to provide medical aid.
He was arrested last year alongside partner Mira Rojkan, a Bulgarian citizen who had been living in Leeds studying law, and she was accused of engaging in terrorist propaganda.
Rojkan was given a suspended sentence, the BBC reported Robinson's mother as saying.
On Saturday, the Kurdistan Solidarity Campaign praised Robinson for "selflessly" battling IS, and called on the Government to stand up for the former soldier.
"Joe is a hero to all who value peace and freedom against the dark forces of oppression, enslavement, rape and terrorism which IS represent," a statement said.
The YPG is not a proscribed organisation in the UK.
The Foreign Office said: "We stand ready to provide consular assistance to a British national in Turkey."