The department said that David Coleman Headley answered the questions of investigators from India over the course of seven days.
Headley pleaded guilty in March to being in on the planning of the November 2008 terrorist attacks in Mumbai, India, as well as subsequent planning to attack a Danish newspaper over cartoons depicting the Prophet Muhammad.
India blames Pakistani extremists for the Mumbai attacks, which left 166 people dead.
In his plea agreement, Headley admitted making surveillance videos and conducting other intelligence gathering for the attack on Mumbai. The U.S. and India say the 10 gunmen in the three-day siege were trained and directed by the Pakistani-based terrorist group Lashkar e Taiba (Army of the Pure).
Headley and his counsel agreed to the meetings and there were no restrictions on the questions posed by Indian investigators, the department said.
Indian law enforcement officials were provided direct access to interview Headley "as part of the cooperation and partnership between the United States and India in the fight against international terrorism," the department said in a statement.
Both countries have agreed not to disclose the substance of the interviews, in order to protect the confidentiality of the investigations being conducted by India and the U.S., the Justice Department statement added.
Last week, U.S. and Indian officials wrapped up strategic high-level talks aimed at easing Indians' fears that their country is slipping behind rivals China and Pakistan in U.S. foreign policy priorities.
The Obama administration tried to use last week's meetings to reassure India that it is an important player in many of the global issues the United States wants solved.
India is seen as crucial to the U.S.-led fight against extremists in Pakistan and Afghanistan, as a counterweight to powerful China and as a big part of settling world trade and climate change deals.
President Barack Obama will visit India in early November.
The Bush administration in 2008 pushed through a landmark accord to establish civilian nuclear trade with India, transforming ties after decades of mistrust.
As part of Headley's plea bargain agreement, the U.S. government agreed not to extradite him to India, Pakistan or Denmark for the charges for which he has admitted guilt.
Headley was born in the U.S. to a Pakistani father and American mother. They moved to Pakistan.
According to court documents, Headley changed his name in 2006 to get across international boundaries without too many questions.
In the plea agreement, Headley told how he met with terrorist leader Ilyas Kashmiri in Waziristan in the tribal areas of western Pakistan in May 2009. He said Kashmiri put him in touch with a European contact who could provide weapons and manpower for the Danish attack.