This marks the first role Neeson has officially accepted since the death last month of his wife, actress Natasha Richardson. The deal reportedly was in the works before Richardson died of a head injury she sustained in a skiing accident.
Fiennes and Richardson appeared together in the films "Maid in Manhattan" and "The White Countess." He reportedly was one of the mourners who attended her funeral.
The Hollywood Reporter said Louis Leterrier will direct the new version of "Titans" with Neeson set to play Zeus and Fiennes in negotiations to portray Hades.
The "Titans" cast also includes Sam Worthington, Liam Cunningham, Mads Mikkelsen, Gemma Arterton and Alexa Davalos.
It is slated to start shooting later this month in the United Kingdom, the entertainment industry trade newspaper said.