The once-powerful Republican lobbyist entered the plea one day after he owned up in a Washington court to a multimillion-dollar fraud.
In both cases, Abramoff, 46, entered a plea deal with the government and agreed to cooperate in ongoing investigations of lawmakers who may be implicated in his web of influence-peddling.
On Wednesday, he admitted he used fake documents to obtain a 60-million-dollar loan for the purchase of a fleet of gambling boats in 2000.
He pleaded guilty to wire fraud and conspiracy to commit wire and mail fraud.
"He will face the consequences of his actions," said US Attorney Alex Acosta said after Judge Paul Huck set sentencing for March 16.
"He will face, on orders of the court, significant jail time," Mr Acosta told journalists outside the Miami courthouse.
Plea bargain
As part of the plea deal, Abramoff could face six years in prison, about five times less than the maximum sentence he initially risked.
The deal makes it clear the US attorney's sentencing recommendations could increase or decrease depending on Abramoff's cooperation.
Under the plea-deal, the lobbyist agreed to provide information, testimony and evidence, and to appear at judicial proceedings if required.
An associate of Abramoff’s, Adam Kidan, also pleaded guilty in December for his part in the fraudulent 2000 deal.
The two admitted they faked documents to falsely demonstrate they had contributed 23 million US dollars for the purchase of the SunCruz line from Miami businessman Konstantinos Boulis.
Citadel Equity had agreed to fund 60 million US dollars in loans toward the 147.5 million US dollar deal provided that the buyers made a cash equity contribution of at least 23 million US dollars.
A few months after the sale, Boulis was slain gangland-style. Three men were arrested in connection with the murder last year.
Abramoff and Kidan both denied any involvement in the killing.
Mr Acosta has declined to say whether the murder was being probed as part of wide ranging investigations into Abramoff's activities.
The former high-flying lobbyist arrived at the courthouse in a limousine, wearing a dark suit and a baseball cap.
He left discreetly through a side door after his lawyers complained he was almost knocked over by journalists.
On Tuesday, Abramoff spoke briefly outside the US District Court in
Washington to express "profound regret and sorrow" for what he called the "mistakes" he committed.
Superbowl tickets
He admitted fraudulently taking millions of dollars from Indian tribes he was paid to represent.
He also said he obtained agreements from Congressmen and staff to pass legislation in return for trips to famous Scottish golf links, Superbowl tickets and meals at a swank Washington restaurant.
The case could lift the lid on political backroom deals and influence peddling in Washington.
Up to 20 members of Congress are reported to be in the sights of prosecutors, including Tom Delay, the former majority leader in the House of Representatives.
The scandal also has the potential to wound President George W Bush's Republican Party ahead of Congressional elections.
White House spokesman Scott McClellan Wednesday told reporters that Mr Bush did not know Abramoff, but that the money the lobbyist raised for the president's re-election campaign would be donated to charity.
Democrats say Abramoff raised more than 100,000 US dollars for Mr Bush, while Mr McClellan put the figure at "several thousands" of dollars.