Scott Fenstermaker, the lawyer for accused terrorist Ali Abd al-Aziz Ali, said the men would not deny their role in the 2001 attacks but "would explain what happened and why they did it."
The U.S. Justice Department announced earlier this month that Ali and four other men accused of murdering nearly 3,000 people in the deadliest terrorist attack in the U.S. will face a civilian federal trial just blocks from the site of the destroyed World Trade Center.
Ali, also known as Ammar al-Baluchi, is a nephew of professed 9/11 mastermind Khalid Sheikh Mohammed.
Mohammed, Ali and the others will explain "their assessment of American foreign policy," Fenstermaker said.
"Their assessment is negative," he said.
Fenstermaker met with Ali last week at the U.S. prison at Guantanamo Bay in Cuba. He has not spoken with the others but said the men have discussed the trial among themselves.
Fenstermaker was first quoted in The New York Times in Sunday's editions.
Critics of Attorney General Eric Holder's decision to try the men in a New York City civilian courthouse have warned that the trial would provide the defendants with a propaganda platform.
Dean Boyd, a spokesman for the Department of Justice, said Sunday that while the men may attempt to use the trial to express their views, "we have full confidence in the ability of the courts and in particular the federal judge who may preside over the trial to ensure that the proceeding is conducted appropriately and with minimal disruption, as federal courts have done in the past."
Last week, the Senate Judiciary Committee questioned Holder for hours about his decision to send the five 9/11 suspects to New York for trial.
Critics of Holder's decision - mostly Republicans - argued the trial will give Mohammed and his co-defendants a world stage to spout hateful rhetoric. Holder said such concerns are misplaced, and any pronouncements by the suspects would only make them look worse.
"I have every confidence that the nation and the world will see him for the coward that he is," Holder told the committee. "I'm not scared of what Khalid Sheikh Mohammed has to say at trial - and no one else needs to be, either."
The attorney general said he does not believe holding the trial in New York - at a federal courthouse that has seen a number of high-profile terrorism trials in recent decades - will increase the risk of terror attacks there.