The US Army said the soldier, identified by a court in Vladivostok as Gordon Black, had been detained on criminal charges.
It withheld details, citing the sensitivity of the case.
The detention of Black adds to the number of US citizens held in Russia at a time of tensions with Washington over Moscow’s invasion of Ukraine.
Primorye regional court spokeswoman Elena Oleneva said Black was detained on May 2 and would be held in pre-trial detention until July 2 for theft, state-run Ria Novosti news agency reported.
Vladivostok city police said they had arrested a 34-year-old foreigner and opened a criminal case over theft causing significant harm to the victim, punishable by up to five years in prison.
The suspect is being held in a pre-trial detention centre in the city, they said.
Police said the man had begun a relationship with a Russian woman while she was working in South Korea.
They kept in touch online and he came to visit her on April 10.
The couple had a “conflict” and the man left.
Afterwards, the woman found money was missing and called police, who tracked down the man in a hotel, as he was preparing to fly out to the United States.
Russian newspaper Izvestia reported earlier, citing a source, that the man had stolen 200,000 rubles ($2,200) and beaten the woman up.
