The 72-year-old Weiss entered his plea under a previously announced agreement with prosecutors. He has been ordered to pay nearly $10 million in fines and forfeiture penalties, and could be sentenced to up to 33 months in prison at a later hearing.
Asked by U.S. District Judge John F. Walter if he was pleading guilty, Weiss said, "Yes I am."
"I take responsibility for everything," said Weiss, who wore a navy pinstriped suit. "My direct participation differed as to each" incident.
Federal prosecutors have said they will ask Walter to impose the full 33-month term.
Attorney Benjamin Brafman, who represents Weiss, said he was hopeful the court would consider during sentencing that Weiss had acknowledged the criminal conduct and remained "one of the true legal giants of his generation."
Prosecutors have said the Milberg Weiss firm made an estimated $250 million over two decades by filing legal actions on behalf of professional plaintiffs who received $11.3 million in kickbacks.
The firm dominated the industry in securities class-action lawsuits, which involve shareholders who claim they suffered losses because executives misled them about a company's financial condition.
The kickback scheme allowed the firm's attorneys to be among the first to file litigation and secure the lucrative position as lead plaintiffs' counsel, according to court documents.
The firm's lawsuits targeted companies such as AT&T Inc., Lucent, WorldCom, Microsoft Corp. and Prudential Insurance.
Prosecutors said Seymour Lazar, 80, was paid about $2.6 million to be a professional plaintiff and help the law firm, previously known as Milberg Weiss Bershad & Schulman, in its pursuit of the lawsuits.
The retired attorney was ordered to spend six months in home detention and two years probation. He also was fined $600,000 after pleading guilty to obstruction of justice, subscribing to a false tax return and making a false declaration to the court.
The seven-year investigation also resulted in guilty pleas by three of Weiss' former partners.
William Lerach, whose high-profile legal victories included a $7 billion judgment against now-defunct energy company Enron Corp., pleaded guilty to one count of conspiracy to obstruct justice and make false statements. He was sentenced to two years in federal prison.
Steven Schulman pleaded guilty to a racketeering conspiracy charge. He agreed to forfeit $1.85 million to the government and to pay a $250,000 fine.
Former partner David Bershad pleaded guilty to conspiracy and agreed to cooperate with the government.
Schulman and Bershad are scheduled to be sentenced later this year.
With Weiss' guilty plea, there are two defendants remaining in the case the firm itself and attorney Paul T. Selzer. Trials for those defendants are scheduled in August.