The Maharashtra Control of Organised Crime Act (MCOCA) court convicted gangster Rajan in the same case earlier in the day.
Journalists Jigna Vora and Joseph Paulsen were also acquitted by the court in the case.
The verdict comes nearly seven years after Dey was shot dead in suburban Mumbai.
Rajendra S. Nikhalje alias Chhota Rajan, who according to reports had sanctioned Dey's murder, is currently lodged in Tihar Central Jail in New Delhi.
Dey had written two books: 'Khallas: An A to Z Guide to the Underworld' and 'Zero Dial: The Dangerous World of Informers'. (ANI)