The Mali international has joined Orestis Karnezis in signing for the Hornets, while Marvin Zeegelaar also completed a move on deadline day
Watford have further bolstered their defence with the signing of Molla Wague on a season-long loan.
Wague moves to Vicarage Road from Udinese having only joined the Serie A club on a permanent basis in July.
Hornets 23/20 to win or draw at St Mary's
The Mali international spent last season on loan at Leicester City but made just one appearance after dislocating his shoulder in his debut in the FA Cup against Millwall.
He has 25 caps for Mali, playing at three separate Africa Cup of Nations tournaments for Les Aigles.
Earlier on Thursday, Watford signed defender Marvin Zeegelaar on a four-year deal and Udinese goalkeeper Orestis Karnezis on loan.