The plains, foothills and lower hills of the district were part of the
Chutia kingdom while the higher hills were inhabited by Mishmis. Later, the
Khamti people migrated in the late 18th century and settled in the Namsai plains. The Tengaponia clan of the Deoris (priests of Chutia kingdom) had their temple near the Tengapani river of Namsai. The creation of Namsai district was approved by the Arunachal Pradesh government of
Nabam Tuki on 21 March 2013. On 25 November 2014, the Namsai subdivision of Lohit district was declared as a new district of Arunachal Pradesh, becoming the 18th district of the state. ==Demographics==