Adlene Guedioura has departed Watford for fellow Premier League club Middlesbrough in a reported £3.5 million deal on Tuesday.
The Algeria international spent just under 17 months with the Hornets after previously enjoying two separate loan spells at Vicarage Road.
Guedioura was never a regular at the club, making just 12 league starts in the top flight, nine of which have come under Walter Mazzarri's stewardship this season.
The 31-year-old has signed a two-and-a-half-year deal at the Riverside and could make his debut at Tottenham on Saturday.