Harju County Court ruled that Vladimir Poljakov, 34, can be extradited to Ukraine to face terrorism charges for fighting on the side of the separatist forces.
The ruling is subject to appeal and a final decision will be made by the government.
Poljakov denied the charges, saying he was in Ukraine for other reasons.
“Our position is that this is a political crime and under European convention, he cannot be extradited to Ukraine,” Sven Sillar, Poljakov's lawyer, said.
After an investigation by the Internal Security Service (ISS), Poljakov was charged with belonging to a terrorist organization, possession of illegal firearms and ammunition, and driving under the influence of illegal drugs.
According to the charge, Poljakov joined the separatist forces fighting the Ukrainian government in the east of the nation, in June last year. ISS said he was assigned to convoy and guard prisoners, as well as taking part in battles.
ISS apprehended Poljakov in February, when he was allegedly on his way back to Ukraine, via Russia, with authorities finding illegal firearms and ammunition on him.
Poljakov had previously killed an ethnic Russian man in Estonia, allegedly for crossing a street too slowly. He fled Estonia and joined the French Foreign Legion, before returning to Estonia and serving his sentence.
Estonian and Ukrainian authorities cooperated closely in the investigation.
Editor: J.M. Laats