Striker Troy Deeney, linked in British media reports with a move to champions Leicester City, has signed a new five-year contract at Vicarage Road.
Kabasele, a 25-year-old centre half born in the Democratic Republic of Congo, has agreed a five-year contract.
The Belgian is the second player to join Watford this week following the signing of 19-year-old striker Jerome Sinclair from Liverpool.
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British media reported that Granada forward Isaac Success is also poised to move to Watford.
The arrival of Success could see the departure of Odion Ighalo who joined Watford from Granada in 2014.
British media reported that Ighalo could be on his way to China following a 37.5 million-pound($50 million) bid from Shanghai SIPG.
Watford defender Gabriele Angella has returned to Udinese.
(Reporting by Ian Rodricks in Bengaluru and Neil Robinson in London, Editing by Ed Osmond)
