The Proud Boys' former leader Enrique Tarrio has been jailed for 22 years for orchestrating the attack on the US Capitol on 6 January 2021.
It is the longest sentence handed down so far over the attack, which happened as lawmakers were certifying President Joe Biden's 2020 election victory.
Tarrio, 39, was not in Washington during the riot, but helped organise the far-right group's involvement.
As he was led from court, he flashed the two-fingered peace or victory sign.
The Department of Justice's sprawling investigation into the riot has so far seen more than 1,100 people arrested and charged.
The rioters turned out in support of then-president Donald Trump, who continues to deny that he lost the 2020 election. He has promised to pardon most or all of the rioters if he is re-elected president in 2024.
Tarrio was convicted in May of seditious conspiracy, a rarely used charge of planning to overthrow the government, and multiple other counts. He has been in jail since his arrest last year.
In their sentencing recommendation, prosecutors described Tarrio as a "naturally charismatic leader" and "a savvy propagandist" who was the "primary organiser" of the conspiracy he and his co-defendants were convicted of.
They also said he condoned and promoted violence from others. "He was a general rather than a soldier," prosecutors wrote.
They argued he helped rally members of the far-right group to come to Washington DC and, while he was not in the city at the time, prosecutors said he monitored their movements and encouraged them as the attack unfolded.
As Trump supporters laid siege to the congressional complex, Tarrio posted online that he was "enjoying the show".
"Do what must be done," he wrote, urging on the rioters.
US District Court Judge Timothy Kelly, a Trump nominee who presided over the sentencing hearing, concluded that Tarrio began planning an attack on the Capitol in December 2020 and instituted a rigid command structure.
"Tarrio was the ultimate leader, the ultimate person who organised, who was motivated by revolutionary zeal," Judge Kelly said. "I don't have any indication that he is remorseful for the actual things that he was convicted of."
Before he learned his fate on Tuesday, an emotional Tarrio apologised to police and residents of Washington DC for his role in the riot. "I am extremely ashamed and disappointed that they were caused grief and suffering," he said. "I will have to live with that shame for the rest of my life."
Tarrio, who wore an orange jail uniform, added: "I was my own worst enemy. My hubris convinced me that I was a victim and targeted unfairly."
Acknowledging that Mr Trump had lost the November 2020 presidential election, Tarrio said: "I am not a political zealot. I didn't think it was even possible to change the results of the election.
"Please show me mercy," Tarrio asked the judge. "I ask you that you not take my 40s from me."
Tarrio was national chairman of the Proud Boys. Founded in New York City in 2016, members of the far-right group have described themselves as an all-male drinking club.
They regarded themselves as Mr Trump's foot-soldiers and have often been involved in street clashes with far-left anti-fascist activists.
Tarrio's lawyer argued in court on Tuesday that his client was a "keyboard ninja" and "misguided patriot" who tended to "talk trash", but had no intention of overthrowing the government.
However, Judge Kelly noted that Tarrio had on many previous occasions expressed no remorse for his actions.
Tarrio was also found guilty in May of obstruction and conspiracy charges, civil disorder and destruction of government property.
Prosecutors had called his actions "a calculated act of terrorism", meriting a sentence of 33 years in prison. The defence wanted no more than 15 years.
Tarrio stood silently while the judge handed down the penalty. As he was led from court, Tarrio waved to his family in the public gallery and raised the two-fingered salute.
His lawyers said he plans to appeal.
Tuesday's was the last in a series of sentencing hearings for the ringleaders of the Capitol riot.
Until now, the longest sentences were the 18-year terms handed down last week to another Proud Boy, Ethan Nordean, and in May to Stewart Rhodes, founder of the Oath Keepers, a far-right militia.
Three other Proud Boys received prison sentences last week for their roles in the riot.
Former US Marines Dominic Pezzola and Zachary Rehl received 10 and 15 years respectively.
Joe Biggs, a US Army veteran, got 17 years.
The charges against the rioters have varied - from relatively minor crimes like entering a restricted area, to destruction of government property, assault and conspiracy. Around 200 have pleaded guilty to felony charges.
The investigation is ongoing and the FBI is still trying to locate 14 rioters captured on video assaulting police officers or members of the media.
Sign up for our morning newsletter and get BBC News in your inbox.