The Algeria international has not re-joined the Hornets for their pre-season preparations after playing for his country at the World Cup in Brazil this summer.
Watford head coach Beppe Sannino confirmed after Saturday's friendly win over St Albans that Belkalem has not been training with Watford.
And it appears that the 25-year-old is set to join ex-Algeria manager Vahid Halilhodzic at Turkish side Trabzonspor.
Belkalem initially joined the Hornets on a season-long loan deal from sister club Granada at the start of the 2013-14 campaign, with a view making his stay a Vicarage Road permanent; the club confirmed to the that they had taken up that option earlier this season.
Belkalem, who made his debut in the 2-0 League Cup win over AFC Bournemouth, made just 10 appearances during his first season at Vicarage Road as he struggled to shake off an ankle injury.
He was, however, named in Algeria's 23-man squad for this summer's World Cup Finals in Brazil, and impressed for his country as they made it to the knock-out stages before losing to eventual winners Germany in extra-time.
