The centre back has asked to leave the club due to personal reasons.
Chelsea's Diego Costa in action with Sunderland's Younes Kaboul
The 30-year-old joined The Black Cats from Tottenham last summer on a four-year deal, but has told new manager David Moyes he want to leave Wearside with the club accepting a bid from Watford for the centre back.
Sunderland confirmed on their website late Thursday night about Kaboul's decision to leave the Stadium of Light and the former Portsmouth defender paid tribute to the club to allow him to leave after just one season, with Watford yet to discuss personal terms with Kaboul.
Sunderland's Younes Kaboul and Lee Cattermole look dejected
“This is not an easy situation for me, but the club and the manager have shown an understanding of my personal circumstances and I am grateful for that," he said.
“The Sunderland fans have supported me fantastically well since I came to the club, they gave me my passion back and I sincerely thank them for that.
"I hope they understand that I have made this incredibly difficult decision for me."
Sunderland confirmed that Kaboul has been granted permission to speak to The Hornets, with his desire to move back down south a major factor in his decision to leave.
Was told earlier this week Younes Kaboul wanted to head back south. Fair play to #SAFC for putting that first and allowing him to do so.— Kristan Heneage (@KHeneage) August 18, 2016
Kaboul's departure is a major loss to Moyes just one weekend after the start of the Premier League season, with his partnership with Lamine Kone, who has also asked to leave the club due to contractual reasons, pivotal in Sunderland surviving relegation under new England manager Sam Allardyce.
The former Auxerre defender was brought to England courtesy of Tottenham in 2007 where he lasted just one season, moving to Portsmouth a year later under Harry Redknapp.
Two-and-a-half year later, Redknapp took Kaboul back to White Hart Lane when he became manager of Spurs and went on to make 183 appearances in North London before joining Dick Advocaat's Sunderland in 2015.
Kaboul departs having made 24 appearances under three different managers, with his last appearance coming in The Black Cats' 2-1 defeat to Manchester City last weekend.
Lamine Kone celebrates scoring the third goal for Sunderland with Younes Kaboul and Jermain Defoe