The big defender made just 12 appearances in two years after former manager Gary Megson paid Watford £2million for him in August 2008.
And, with 12 months of his contract left, the club have opted to pay off the Nigerian international.
Shittu, who turned 30 today, rejected a move to Turkish outfit Kayserispor earlier in the summer.
But his Reebok days were clearly numbered after failing to feature since Owen Coyle took over in January, and the manager will be delighted to get the centre-half off the wage bill.
Mustapha Riga also looks to be heading for the exit door after being left out of the 24-man squad Wanderers had to submit to the Premier League yesterday by 5pm.
Like Shittu, the 28-year-old forward has been a peripheral figure at the Reebok since joining from Spanish club Levante in July 2008, making just three starts in that time.
Out-of-favour Jlloyd Samuel and goalkeeper Ali Al-Habsi, who is on a season-long loan at Wigan, have been named in a squad which could have been up to a maximum of 25 players, but had to include eight “home-grown” players who have been registered with an English or Welsh club for at least three years between their 16th and 21st birthday.
Coyle will also be able to call on youngsters like Marcos Alonso, Rodrigo Moreno, Danny Ward and Rob Lainton, who do not need to be listed because they are under 21.