The 25-year-old man from Felixstow, in Adelaide's east, was charged by Australian Federal Police with working for a foreign military without authorisation.
It is the first time someone has been charged with the offence, with the man facing up to two decades in jail if found guilty.
Australian laws limit the work defence personnel can perform with a foreign military, government or company without authorisation.
The man allegedly travelled to Ukraine in May 2025 and returned to Australia in January 2026.
AFP investigators executed a search warrant at a property on Thursday, seizing electronic devices, including a mobile phone and laptop.
Forensic examination of the devices allegedly revealed images showing the man participating in a foreign conflict.
The man is due to face Adelaide Magistrates Court on Tuesday on one count of a restricted individual working for a foreign military organisation or government body.
The maximum penalty for the offence is 20 years' jail.
AFP Counter Terrorism Commander Paula Hudson said the agency worked closely with its local, Commonwealth and international partners to protect Australia's interests.
In May 2025, Melbourne man Oscar Jenkins was convicted by a Russian court after allegedly joining the fight in Ukraine.
Mr Jenkins was captured in occupied eastern Ukraine and taken into Russian custody in December 2024, believed to be the first Australian combatant reportedly captured by Russian forces.
At the time, the Australian government insisted Mr Jenkins was fighting as a member of the Ukrainian defence forces and should be entitled to humane treatment as a prisoner of war.
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