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.
According to the 2012 Tanzania National Census, the population of Masasi District was 247,993 and in Masasi town the population was 102,696.
The Masasi District is administratively divided into 34 wards:
- Masasi High Quality Farmers' Products, a cashew growers' association.