Festival organizers had previously announced that Neeson would attend, but a spokesman for the actor said he was never scheduled to do so.
"I'm 110 percent sure he is not attending the festival," said Charlene Coy, publicist for E1 Entertainment, which is representing "Chloe."
"They didn't give us a reason."
Neeson is said to be busy filming "The A-Team" in Vancouver, and the scheduling conflict won't allow him to attend the film fest.
But some speculate it's the Ballymena born actor's grief that's keeping him from traveling to Toronto.
Neeson was filming "Chloe" in the city when his wife, Natasha Richardson, suffered a head injury in a ski accident in Montreal this past March. The Irishman left the set to be by his wife's hospital bedside in New York, where she tragically died at the age of 45.
But he has only made small-scale public appearances since his wife's death, attending the opening night of "Mary Stuart" on Broadway, and walking the red carpet for the New York City premiere of the Irish film "Five Minutes of Heaven."
Reporters at the "Five Minutes" premiere were under strict instructions not to ask the Irish actor any personal questions.
Liam Neeson Plans Not to Attend Toronto 'Chloe' Premiere
The Toronto International Film Festival, which takes place September 10 through 19, would have been Neeson's first major public event since the tragedy, and he would have been bombarded with questions from journalists.
Neeson's no-show at the festival adds to the mysterious cloud surrounding Atom Egoyan's "Chloe."
The film still has not been previewed for the press, which critics say is an uncharacteristic move for Egyptian director Egoyan.
"Chloe" is a thriller about a doctor named Catherine (Moore) who tests her husband David's (Neeson) fidelity by hiring a prostitute named Chloe (Amanda Seyfried) to tempt him and then report back his actions.
The film will screen at the Toronto Film Festival on Sunday, September 13.