This study presents the results of craniometric analysis of 500 Byzantine Era dog skulls from the ancient site of the Theodosius Harbour in Istanbul (modern Yenikapi neighbourhood). To determine the typology of the skulls, up to 36 craniometric measurements were taken on each specimen. Fourteen indices and ratios were calculated from the measurements. Results show that the majority (97%) of the Byzantine dogs from the Yenikapi excavations were of mesocephalic type. Only 15 of the analysed skulls (3%) were of the dolichocephalic type. Only one dog skeleton has been recovered in situ during the excavations; analysis indicates that this individual belonged to a mesocephalic type dog. Brachycephalic dog skulls were absent in the samples studied from Byzantine Yenikapi. The results give important insight in continuities and discontinuities in dog-breeding traditions from the Roman to the Byzantine Eras, and between the West and the East during Medieval times.