The 25-year-old has returned to his homeland to link up with Rene Weiler’s side – who won their 34th Belgian First Division A and will remain at the Constant Vanden Stock Stadium until the end of the 2017/18 campaign.
Belgium international stopper Sels joined United from Gent in June 2016, having played Champions League football and been voted Belgium’s Goalkeeper of the Year during his time with De Buffalo’s.
Sels began last season in goal for the Magpies, making nine appearances in the Sky Bet Championship, keeping five clean sheets.
He also had three outings in the FA Cup, and a further two in the EFL Cup as Newcastle reached the last eight.