Troost-Ekong spent the second half of last season on loan in Italy with Salernitana, but could be on the move once again this summer.
Salernitana had an option to buy included in the loan agreement, however the defender is poised to head to Greece and PAOK Salonika.
The northern Greece side have an agreement with Watford to sign him for a fee of €1m, according to Greek outlet Sportime.
Troost-Ekong was born in the Netherlands, but turns out for Nigeria on the international stage.
Watford signed the centre-back from Udinese in 2020 and during his time at Vicarage Road he has made 68 appearances.
Troost-Ekong clocked 15 outings in the Championship for Watford last term before he was shipped to Italy.
The defender, who played nine times in Serie A, including against heavyweights Juventus, Inter and Roma, is set to join a side that finished fourth in the Greek Superleague last term.
