The federal Bureau of Prisons confirmed that the New York Republican was in custody on Friday at the Federal Correctional Institution in Fairton, in southern New Jersey.
Santos pleaded guilty last summer to federal wire fraud and aggravated identity theft charges for deceiving donors and stealing people's identities in order to fund his congressional campaign.
His lawyer Joe Murray, when asked for comment on Friday, responded with a brief, all-caps text: "FREE GEORGE SANTOS."
The ever-online Santos, who turned 37 on Tuesday, hosted a farewell party for himself on X on Thursday night.
"Well, darlings … The curtain falls, the spotlight dims, and the rhinestones are packed," he wrote in a post afterwards.
"From the halls of Congress to the chaos of cable news what a ride it's been! Was it messy? Always. Glamorous? Occasionally. Honest? I tried … most days."
Santos will serve his time in a minimum security camp at the all-male facility, which also includes a larger medium security prison, according to the Bureau of Prisons.
Santos was elected in 2022, flipping a wealthy district representing parts of Queens and Long Island for the GOP. But he served for less than a year and became just the sixth member of the House to be ousted by colleagues after it was revealed he had fabricated much of his life story.
During his winning campaign, Santos painted himself as a successful business owner who worked at prestigious Wall Street firms when, in reality, he was struggling financially.
He also falsely claimed to have been a volleyball star at a college he never attended and referred to himself as "a proud American Jew" before insisting he meant that he was "Jew-ish" because his Brazilian mother's family had a Jewish background.
The cascade of lies eventually led to congressional and criminal inquiries into how Santos funded his campaign and, ultimately, his political downfall.
Since his ouster from Congress, Santos has been making a living hosting a podcast called "Pants on Fire with George Santos" and hawking personalised video messages on Cameo.
He has also been holding out hope that his unwavering support for President Donald Trump might help him win a last-minute reprieve.
The White House said this week that it "will not comment on the existence or non-existence" of any clemency request.
In media appearances this month, the former lawmaker wasn't shy about sharing his morbid fears about life behind bars.
"I'm not trying to be overdramatic here. I'm just being honest with you. I look at this as practically a death sentence," Santos told Tucker Carlson during an interview.
"I'm not built for this."
On social media, his recent musings have sometimes taken a dark turn.
"I'm heading to prison, folks and I need you to hear this loud and clear: I'm not suicidal. I'm not depressed. I have no intentions of harming myself, and I will not willingly engage in any sexual activity while I'm in there," Santos said on X.
"If anything comes out suggesting otherwise, consider it a lie … full stop."
