Ishinosuke Uwano, who is now 83, fought during World War II on the island of Sakhalin in Russia's Far East before vanishing without a trace.
Uwano reportedly moved to Ukraine in 1965, but had no contact with his Japanese relatives until now.
It is not clear why Uwano remained in Russia, or how he ended up in Ukraine, where he is married with three children.
Uwano is traveling to Japan on April 19 to visit remaining members of his family.
The Japanese government believes about 400 Japanese soldiers from World War II still live in the Soviet Union's successor states.