FSB claims that Roman Suschenko is a colonel in Ukraine's military intelligence service, EFE news reported.
Media, citing Russian security agencies, said Suschenko of the Chief Directorate of Intelligence of the Ministry of Defence of Ukraine was arrested during an anti-espionage operation in Moscow.
FSB accused Suschenko of collecting information that contained state secrets about the activities of Russian armed forces and the national guard, thereby putting the country's defence capacities at risk.
Russian authorities have ordered a criminal investigation against the detained for spying.
Suschenko, 47, is a correspondent of Ukrainian National News Agency Ukrinform and news of his arrest on Friday broke when a Russian non-profit came across him at the Lefortovo Prison in Moscow.
Ukrinform confirmed Suschenko's arrest and said he had travelled to Moscow on vacation.
The news agency added that the journalist was arrested the same day he arrived at the Russian capital and neither his family nor the Ukrainian consulate were informed of his arrest, which is a violation of international law.
The agency called Suschenko's arrest a planned provocation and said he had been working in Ukrinform since 2002 and been its correspondent in France since 2010.
Meanwhile, First Deputy Chairman of the Verkhovna Rada (Parliament) of Ukraine Iryna Gerashchenko, called for the journalist's immediate release.
