The Belgium international passed his medical and signed a five-year contract with the Jupiler Pro League side on Monday, ending his six-year spell at Anfield.
His fee is an initial £6.4m, rising to £8m with achievable add-ons.
Liverpool are set to bring in free agent Adrian to be the club's new No. 2, sources told ESPN FC.
The 32-year-old Spaniard, who left West Ham after six years this summer, was due to have talks with Real Valladolid before receiving an offer from the Reds.
A transfer is expected to be completed soon given Liverpool kick off the new Premier League season on Friday and would like to have experienced backup to Alisson in place for the opener against Norwich City