Diocletianopolis (Greek: Διοκλητιανούπολις, Bulgarian: Диоклецианопол, "Town of Diocletian") was an ancient Roman town in the region of Thrace, nowadays the town of Hisarya in Bulgaria. It gained its official status as a city in 293 CE by the Roman emperor Diocletian.
Archeological research shows that the place was inhabited 6,000 years BCE. Later, a Thracian settlement developed there. About the 1st century CE Thrace was conquered by the Roman Empire and the settlement became a typical Roman city.
In 293 CE the Roman emperor Diocletian came here because of the healing mineral springs and the town was named after him.