The highly-rated midfielder will play at Vicarage Road for the rest of the season after agreeing to a loan move. Watford also have the option to buy Ndong permanently in the summer.
The 23-year-old joined Sunderland for a club record fee in August 2016 from French side Lorient and has made 54 appearances for the 'Black Cats', 31 of which came in the Premier League last season.
That experience at the top level will be indispensable for new manager Javi Garcia who was appointed on 21st January and is still finding his feet in England.
January 31, 2018