The man, named as US diplomat Ryan Fogle, was held overnight before being released to US officials, Russia’s Federal Security Service (FSB) said.
Russia has declared him “persona non grata” and ordered his expulsion, the foreign ministry said on its website.
Video footage and photographs have emerged of his alleged detention.
The agent was reportedly arrested with a large sum of money, technical devices and written instructions for the agent he had tried to recruit, as well as a number of wigs.
Steve Rosenberg reports from Moscow.