The 24-year-old Norwegian, named as Ishaq Ahmed by the TV2 channel, left for Syria in October 2013. Prosecutors say he fought for IS before joining the Al-Nusra Front, Al-Qaeda's Syrian affiliate. He could face up to nine years in jail if found guilty of "support for a terrorist organisation".
Photographs posted on Facebook showed him posing with weapons.
Ahmed insists he is innocent and that he was only in Syria to carry out humanitarian work. But prosecutors maintain that in the months before his departure he carried out online research into weapons and body armour.
"We have found no evidence of humanitarian work or an effort to procure medical materials," prosecutor Geir Evanger said, quoted by the NTB news agency.
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Ahmed was shot in the leg and crossed the border into Turkey at the beginning of 2014, contacting the Norwegian embassy and demanding to be repatriated.
His is the second trial in Norway over suspected links to the IS militants who have seized swathes of Iraq and Syria. The first trial in May saw three men, aged between 25 and 30, receive jail sentences ranging from seven months to four years and nine months.