TASS quoted law enforcement agencies as saying the man, whom it named as Park Won-soon, had been detained in the far eastern city of Vladivostok before being transferred to Moscow for "investigative actions".
The state news outlet said it was the first such case against a South Korean national. It did not provide any detail on the nature of the alleged spying.
An official reached at South Korea's Foreign Ministry had no immediate comment.
Russia regards South Korea as an "unfriendly" country because of Seoul's support for Western sanctions against Moscow over the war in Ukraine.
At the same time, Russia has cultivated closer relations with North Korea, which the United States says is providing munitions to Moscow for use in the war. North Korea and Russia have denied this, although they have pledged to strengthen military cooperation.
(Reporting by Mark Trevelyan in London and Jack Kim in Seoul; Editing by Alison Williams)
