Masasi is one of the six districts of the Mtwara Region of Tanzania.

It is bordered to the north by the Lindi Region, to the east by the Newala District, to the south by the Ruvuma River and Mozambique and to the west by Nanyumbu District.

The council has five administrative divisions, 34 wards, 166 villages[4] and 864 hamlets.

It has a total area of 3,829.9 km2 (1,478.7 sq mi), and the physical size is 20.8% of the whole region.

The Masasi District is, as at 2023, administratively divided into 5 divisions and 34 wards:[4][8]