This study investigates the provenance of white and colored marbles sampled from architectural elements of the octagonal building at ancient Gadara (Umm Qais), Northern Jordan. The octagonal building dates to the Roman times and was used in later periods. The samples were described macroscopically, then analyses of petrographic thin sections, stable isotopes of oxygen and carbon and major and some trace elements were carried out. The results were compared to the databases reported for the main marble sources of the Mediterranean used in ancient times. Most of the white marbles are probably from Proconnesus (Turkey), whereas Docimium (Turkey), Naxos and Thasos (Greece) are minor sources. The most probable source of the green marble “cipollino verde” is Karystos (Greece), while the red marbles probably come from Iasos (Turkey).