Thanchi (Bengali: থানচি) is an upazila of Bandarban District in the Division of Chittagong, Bangladesh.

Thanchi upazila is bordered by Ruma upazila and Rangamati district to the north, Lama and Alikadam upazilas to the west, Rakhine State of Myanmar to the south and Chin State of Myanmar to the east.

At the boundary with Ali Kadam Upazila, the 35-kilometre-long (22 mi) Alikadam-Thanchi Road ascends hill Dim Pahar, making it one of the highest motorable roads in Bangladesh.

The ethnic population was 26,103 (87.62%), of which Marma were 9,345, Mru 7,021,Tripura 6,336, Khumi 1,829, Bom 685, Chakma 488 and Khyang 359.

The union parishads are subdivided into 12 mauzas and 178 villages.