Accused Russian agent Maria Butina pleaded guilty to one count of conspiracy on Thursday.
Butina, 30, was arrested in July on charges of being a foreign agent for Russia and trying to influence US policy by getting close to the National Rifle Association (NRA) in the lead-up to the 2016 election.
The powerful gun lobby group is known to have close ties to Republican party politicians, including US President Donald Trump.
Butina has previously maintained her innocence, pleading not guilty to charges that she was acting as a foreign spy for Russia.
Previously a student at the American University in Washington, Butina is a founder of the pro-gun Russian advocacy group Right to Bear Arms.
There are Facebook photos showing her posing for pictures with the NRA.
Prosecutors say she worked with a Russian official and two US citizens to try to infiltrate the NRA lobby group and influence Washington's policy towards Moscow.
Her lawyers have identified the Russian official as Alexander Torshin, a deputy governor of Russia’s central bank who was hit with US Treasury Department sanctions in April.
One of the two US citizens mentioned in the prosecutors' criminal complaint was Paul Erickson, a conservative US political activist who was dating Butina.
However, neither Erickson nor Torshin has been accused by prosecutors of wrongdoing, Reuters reports.