The individual is alleged to have been collecting secret information about "the latest weapons and their secret technical documentation".
Russian security services say they have arrested a Ukrainian citizen on suspicion of stealing state secrets.
The suspect was caught "red-handed" in the western city of Tula, Russia's Federal Security Service (FSB) said in a statement.
The individual is alleged to have been collecting secret information about "the latest weapons and their secret technical documentation".
The FSB added the suspect had been trying to search for "employees of Russian defence enterprises" and recruit them as informers.
"Their goals were recruiting the Russians and obtaining from them legally protected information about promising developments in the field of small arms," the statement said.
Tula is home to a military weapons factory that produces guided missiles, as well as advanced automatic and sniper rifles, according to its website.
Russia also says the suspect had been working for the Ukrainian intelligence services, without giving any more details.
A criminal investigation has been opened into the incident. Ukraine's foreign ministry has not commented on the arrest.
Relations between Kyiv and Moscow deteriorated in 2014 after Russia annexed Crimea from Ukraine and backed pro-Russian separatist fighters. An estimated 14,000 people have been killed in the conflict in the Donbas region in eastern Ukraine.
