Neeson, famed for his roles in the Star Wars prequels, will play the role of the player's father and appear throughout the game.
"This role was written with Liam in mind, and provides the dramatic tone for the entire game," said Todd Howard, executive producer of Fallout 3.
"It's been a pleasure bringing the father to life and working with the wonderfully talented people at Bethesda on Fallout 3," added Neeson. "I hope the fans of the franchise and the game will be excited by the results."
Fallout 3 will be released on next-gen formats in 2008.