As of the last official census in 2011, Naharlagun had a population of 36,158. However, due to rapid urbanization, the establishment of the railway line, and the opening of the medical college, the current population is estimated to be significantly higher, functioning seamlessly with Itanagar's metropolitan spread.
The demographic fabric is highly diverse. The indigenous Nyishi tribe is the predominant local community. However, because Naharlagun is a commercial and transport hub, it has a large population of Apatani, Adi, Galo, and Monpa people, as well as a substantial number of residents from states like Assam, Bihar, West Bengal, and Uttar Pradesh who work in trade, construction, and government services.
Religiously, the city is a mix of Donyi-Polo (the indigenous animistic faith), Christianity, Hinduism, Islam, and Buddhism. Hindi acts as the primary lingua franca connecting the diverse linguistic groups, while English is the language of official government business.