Masasi is a district located in the Mtwara Region of Tanzania. It serves as one of the six administrative districts within this region.
The geographic boundaries of Masasi are defined by several neighboring areas and international borders. It is bordered to the north by the Lindi Region, to the east by the Newala District, and to the west by the Nanyumbu District. To the south, the district is bounded by the Ruvuma River and the country of Mozambique.