Zachlumia or Zahumlje (pronounced [zǎxuːmʎe]) was a medieval principality located in the modern-day regions of Herzegovina and southern Dalmatia (today parts of Bosnia and Herzegovina and Croatia, respectively). While sometimes a fully independent or semi-independent Slavic entity, Zahumlje was mainly under foreign powers; the Byzantine Empire, Kingdom of Croatia, Kingdom of Serbia, Kingdom of Hungary, Kingdom of Bosnia and at the end under the Ottoman Empire.

Zachlumia is a derivative of Hum, from Vulgar Latin (Vlach) culme (Latin: culmen) meaning "Hill". South Slavic Zahumlje is named after the mountain of Hum (za + Hum "behind the Hum"), above Bona, at the mouth of the Buna. The principality is named Zahumlje or Hum in Bosnian, Croatian and Serbian (Serbian Cyrillic: Захумље, Хум). It is Zachlumia in Latin, Хлъмъ in Old Church Slavonic, and Ζαχλούμων χώρα ("land of Zachlumians") in Greek. The names Chelmania, Chulmia and terra de Chelmo appear in later Latin and Italian chronicles.

The Archonty of Hum had two major cities: Bona and Hum. The main settlements in Zachlumia were Ston, Ošlje, Dobar, and the towns of Mokriski and Glumainik. The Principality extended to Dalmatia in the northwest and to Pagania in the west; to the mountain of Kalinovik and the Field of Gatak, where it bordered Travunia. The easternmost border of Zahumlje followed the line Popovo-Ljubinje-Dabar and met the Travunian border at the City of Ragusa, which had to pay the annual tax mogorish of 36 pieces of gold to the Zachlumian rulers and at times accept their rule. Zachlumia was divided into the 9 Zhupanates of Ston, Popovo, Dubrava, Luka, Dabar, Žapska, goričku and Večenik around Neretva. Zahumlje had access to the Adriatic Sea at the Peninsula of Rat and bordered Serbia in the north. In its later periods, Zahumlje was split into two Duchies: Upper Zahumlje in the west and Lower Zahumlje in the east.^{[citation needed]}