The Roman Forum of Thessalonica was discovered a block to the south of the church of Agios Dimitrios. It dates to the second century A.D. but the remains of an earlier (Hellenistic) Agora are probably located below it and/or in the adjacent built up area.
This may have been the area of the "marketplace" where a mob started to riot in the city due to the success of the preaching of Paul and Silas (Acts 17:5).