The singer reached the Judges' Houses period of the ITV1 talent show last year, but was turned down for the final group of hopefuls by Gary Barlow and his guest mentor Robbie Williams.
He is said to have auditioned for this year's judges at London's O2 Arena, and managed to impress the panel.
"Your 'no' was the hardest for me by far and I've been waiting for this moment," Barlow said to Cox, according to The Sun.
"You're going to be a great artist. Joe, welcome back."
Last year, Joe Cox described Gary Barlow as an "inspiring" but "brutal" mentor.
Yesterday, it was revealed that former Popstars: The Rivals contestant Nicola Ward auditioned for this year's show.
Watch Joe Cox during last year's X Factor below: