Top Greek club PAOK paid Watford around a Million Pounds to sign William Ekong on a three-year contract.
The 29-year-old Ekong spent the second half of last season on loan at Serie A club Salernitana, who did not trigger the option to sign him permanently.
The defender has joined PAOK, who finished fourth last season and will face Beitar Jerusalem in the Europa Conference League second qualifying round later this month.
He signed for Watford from Udinese in September 2020 and was a key figure in defence as the Hornets secured promotion to the Premier League with the joint-lowest goals conceded figure.
His game time was more limited in the Premier League, and he made 17 outings as the Hornets were relegated.
Last season Ekong made 15 appearances before heading off to Salernitana, and famously scored the second goal in the 4-0 rout of Luton at Vicarage Road in October before mocking Hatters’ Gabriel Osho by pretending to cry when the defender was sent off seven minutes from the end.