No injuries were reported. The cause of the explosion, which happened around 4 a.m. local time (0300 GMT), was unknown, VRT said.
Liege's mayor Willy Demeyer called the incident an "extremely violent act of antisemitism" in comments made to French language public broadcaster RTBF.
The explosion blew out the windows of the synagogue, as well as those of a building on the opposite side of the road, the broadcaster said.
Police cordoned off the street while a specialised counter-terrorism team investigated the incident.
Local police were not immediately available for comment.
(Reporting by Bart Meijer; Editing by Aidan Lewis)

Police officers secure the area outside a synagogue after an explosion in Liege, Belgium, 09 March 2026. The blast occurred at around 04:00 local time outside the synagogue on Rue Leon Fredericq, shattering windows and causing material damage, while no injuries were reported, as Belgian authorities launched an investigation into the incident. EPA/OLIVIER HOSLET