Kolasib is situated in northern Mizoram, bordered by the Cachar and Hailakandi districts of Assam to the north and northwest. The town rests on a ridge at an elevation of approximately 678 meters (2,224 feet), which is somewhat lower than the state capital, Aizawl.
The physical geography of the surrounding district is characterized by a mix of rolling hills, deep river valleys, and dense tropical evergreen and semi-evergreen forests. Two major rivers flank the region: the Tlawng River (also known as the Dhaleswari or Sonai) to the west, and the Serlui River to the east. These river systems are vital to the area, providing water for irrigation, fishing, and hydroelectric power.
The local environment is rich in biodiversity, typical of the Indo-Burma biodiversity hotspot. The forests surrounding Kolasib are home to various species of bamboo, timber, orchids, and diverse avian life.