Tematagi or Tematangi is an atoll in the southeastern area of the Tuamotu Archipelago, French Polynesia. Tematagi's nearest neighbour is Mururoa, which is located 161 kilometres (100 miles) to the ESE.

Tematagi is a medium-sized atoll. It measures approximately 11.5 kilometres (7.1 miles) in length and has a maximum width of 7 kilometres (4.3 miles) with a land area of almost 8 square kilometres (3.1 square miles). As its reef completely encloses the deep lagoon, which is 61 square kilometres (24 square miles), there is no navigable pass to enter the lagoon.