The Russian citizen was detained December 24 in the Georgian port of Poti during passport control procedures on board a ferry bound for the Russian port of Ilychevsk, the source said.
According to local authorities, the detainee entered Georgia without a visa through the Roksky tunnel that connects North and South Ossetia.
Tbilisi has proposed establishing a joint Russian-Georgian border and customs checkpoint at the southern exit of the Roksky Tunnel, alleging that foreign fighters, illegal weapons and contraband often enter South Ossetia through the tunnel.
Georgia has recently begun to increase control over the movement of Russian citizens, especially military personnel, on its territory.
On December 18, police denied Lieutenant General Valery Yevnevich, the commander of Russia's Land Forces, entry into Abkhazia where he was to join the Commonwealth of Independent States' collective peacekeeping force in the self-proclaimed republic.
The following day, police stopped a car with three Russian military officials and took them into custody.
The Russian Foreign Ministry denounced the arrests as pre-planned acts of provocation aimed at thwarting recent positive advances in the dialogue with Georgia, including on military issues.