A separate Latvian foreign ministry statement said that the individual had "performed actions that are inconsistent with the status of a diplomat".
The statement said that Viktor Kalyuzhny, Russia's ambassador to Latvia, had been called to the foreign ministry to be informed of the decision.
The expulsion comes after Latvian television station TV3 reported on December 30 that Latvia was set to order a Russian embassy first secretary to leave.
Maris Rieksins, Latvia’s foreign minister, denied on Monday that the man being expelled was the first secretary at the Russian embassy.
Latvia and Russia signed an agreement last year to formalise their border, 17 years after the Baltic state regained independence.
However, relations have deteriorated amid Russian allegations of discrimination by Latvia against its large Russian-speaking minority.
Russia has also cut off oil supplies to Latvia's western oil port and has expressed unease about Riga's membership of Nato.