In recent years, few studies have been published concerning the so-called Kingdom of Upper Mesopotamia. This is well understood if we consider the paucity of the available information; nevertheless, in absence of new and fresh data, it is crucial to look back to what we have and continue to offer new possible interpretations. Therefore, this paper deals with the question of the identification of the two renowned fortresses of Dūr-Samsī-Addu and Dūr-Addu, built (or conquered) at the beginning of the 18th century BC by Samsī-Addu, ruler of the Kingdom of Upper Mesopotamia, and later lost to the king of Yamḫad Sūmū-Ēpuḫ, who probably renamed the first fortress after himself (i.e. Dūr-Sūmū-Ēpuḫ). In doing so, the textual and archaeological data available will be analysed, namely, the Old Babylonian cuneiform documents coming from the city of Mari (modern Tell Hariri) and some other contemporary sites (e.g. Tell Leilan), and the excavation reports of several sites located near the Sajur River (e.g. Qala‘at Halwanji) and the Syrian Middle Euphrates (e.g. Tell el-Qitar).