George Papadopoulos has cooperated for more than a year with Robert Mueller's Russia probe.
A former advisor to US President Donald Trump whose contacts with Russians set off the investigation into possible collusion with Moscow was jailed Friday for lying to the FBI.
US District Judge Randolph Moss sentenced foreign policy aide George Papadopoulos to 14 days, noting that he "lied in an investigation that was important to national security."
"In January 2017, I made a terrible mistake for which I paid dearly, I am ashamed," Papadopoulos had told the court in Washington.
"I was young and ambitious."
The judge said he took into consideration Papadopoulos's "genuine remorse" in issuing the light sentence, which included a $9,500 fine, a year on parole and community service.
By lying to investigators, Papadopoulos had made "a calculated exercise of self-interest over the national interest," said Moss.
Papadopoulos has cooperated for more than a year with Special Counsel Robert Mueller's probe into Russian interference in the US presidential election and possible collusion with the Trump campaign.
He has testified that senior members of the Trump campaign encouraged him during 2016 to build ties with Russia, including after he told them that Moscow possessed -- and could be willing to share -- dirt on Trump's election rival Hillary Clinton.
Those issues are now at the heart of Mueller's sprawling investigation, which increasingly threatens Trump's inner circle and the president himself.
Out of the 35 people and entities so far charged, Papadopoulos is one of five who have pleaded guilty and will be the second to be sentenced.
Trump has regularly lashed out against the sprawling probe he dubs a "witch hunt" driven by his Democratic foes.
Papdopoulos's lawyer Tom Breen said "the president of the United States hindered this investigation more than George Papadopoulos ever did."