Olatunbosun Ugbogu, a 23-year-old Nigerian citizen serving in the U.S. Navy, was found guilty of stabbing 61-year-old Masaaki Takahashi to death near a U.S. naval base in Yokosuka, south of Tokyo, in March 2008, according to a Foreign Ministry official who declined to be named, citing department policy.
Ugbogu was initially apprehended by U.S. Navy authorities in Tokyo on a desertion charge, but was handed over to Japanese authorities because a credit card in his name was found in the victim's car.
Presiding Judge Masaaki Kawaguchi ruled that Ugbogu, who testified that he heard "voices" ordering him to kill Takahashi, was mentally competent to stand trial, according to Kyodo News agency.
Japan hosts some 50,000 American troops under a security treaty.
Japanese anger over the U.S. military presence grew last year following the alleged rape of a 14-year-old girl by a U.S. Marine on Okinawa.
Earlier in Yokosuka, a Japanese court convicted a U.S. sailor of robbing and fatally beating a 56-year-old Japanese woman in 2006 and sentenced him to life in prison.