The Iseum is the reconstructed sanctuary of the Egyptian goddess Isis, who was worshipped widely across the Roman world. The original temple stood here in the second century, and the careful modern reconstruction — a mix of preserved relief panels, replaced columns, and new walling — gives a clearer sense of a Roman temple than most ruins manage.
An attached museum displays the original carved reliefs along with finds from Roman Pannonia. It is on Rákóczi Ferenc utca, a few minutes east of the cathedral.