Joe Robinson, 25, from Leeds, spent five months battling the insurgents alongside Kurdish armed forces in 2015.
But he was arrested on a beach while on holiday in Turkey last July and charged with terrorism offences.
Ankara considers the YPG – the armed group Mr Robinson had fought with – an illegal terror organisation.
He was handed a seven-and-a-half year sentence after a trial, which he was not allowed to attend, on Friday.
His student fiancée, Mira Rojkan, who was arrested at the same time, was given a suspended sentence for “terrorism propaganda”. She says her only involvement was to share pro-Kurdish posts on Facebook and YouTube.
Mr Robinson’s mother, Sharon Chimejczuk, said she had been informed of the court’s decisions via a phone call from an official at the British Foreign Office.
The former army medic had previously served with UK forces in Afghanistan in 2012.
He travelled to Syria after becoming increasingly incensed by both Isis propaganda videos and what he saw as the UK’s inaction in the region.
His family had believed he was leaving the country to join the French Foreign Legion.
When he returned to the UK in November 2015 he was arrested at Manchester Airport on suspicion of terrorism offences but all charges were dropped 10 months later.
It is understood he travelled to Turkey on holiday without understanding the country’s stance on the YPG.
A Foreign Office spokesman said: "We stand ready to provide consular assistance to a British national in Turkey."
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