"The 23-year-old will spend the remainder of the 2017/18 season with Watford, while the club also have the option on a permanent transfer in the summer".
He becomes the third signing since Javi Gracia took over as manager, following on from Barcelona forward Gerard Deulofeu and Anderlecht midfielder Dodi Lukebakio.
Sunderland are understood to have secured a loan fee as part of the loan deal.
The 23-year-old switched to the Stadium of Light from Lorient for a fee of £13.6million in August 2016.
Ndong's arrival on Wearside brought plenty of hope to the club, however, the player was unable to prevent Sunderland from falling through the relegation trapdoor last season following a truly dismal campaign.
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