Tamil Nadu (; Tamil: [ˈtamiɻ ˈnaːɽɯ] (listen), abbr. TN) is a state in southern India. It is the tenth largest Indian state by area and the sixth largest by population. Its capital and largest city is Chennai. Tamil Nadu is the home of the Tamil people, whose Tamil language—one of the longest surviving classical languages in the world—is widely spoken in the state and serves as its official language. The state lies in the southernmost part of the Indian peninsula, and is bordered by the Indian union territory of Puducherry and the states of Kerala, Karnataka, and Andhra Pradesh, as well as an international maritime border with Sri Lanka. It is bounded by the Western Ghats in the west, the Eastern Ghats in the north, the Bay of Bengal in the east, the Gulf of Mannar and Palk Strait to the south-east, and the Indian Ocean in the south. The at-large Tamilakam region that has been inhabited by Tamils was under several regimes, such as the Sangam era rulers of the Chera, Chola, and Pandya clans, the Pallava dynasty, and the later Vijayanagara Empire, all of which shaped the state's cuisine, culture, and architecture. After the fall of the Kingdom of Mysore, the British colonised the region and administered it as part of the Madras Presidency, headquartered at the city of Madras, now known as Chennai. After India's Independence in 1947, the Madras State came into existence, whose borders were linguistically redrawn by the States Reorganisation Act, 1956, losing territory to Kerala, Karnataka and Andhra Pradesh. The state was renamed as Tamil Nadu in 1969. It is home to a number of historic buildings, multi-religious pilgrimage sites, hill stations and three World Heritage Sites.The economy of Tamil Nadu is the second-largest in India, with a gross state domestic product (GSDP) of ₹24.85 lakh crore (US$310 billion) and has the country's 11th-highest GSDP per capita of ₹225,106 (US$2,800). It ranks 11th among all Indian states in human development index. Tamil Nadu is the most urbanised state in India, and one of the most industrialised states; the manufacturing sector accounts for more than one-third of the state's GDP. Its tourism industry is the largest among the Indian states. The Tamil film industry plays an influential role in the state's popular culture.

The Nilgiri Mountains form part of the Western Ghats in northwestern Tamil Nadu, Southern Karnataka, and eastern Kerala in India. They are located at the trijunction of three states and connect the Western Ghats with the Eastern Ghats. At least 24 of the Nilgiri Mountains' peaks are above 2,000 metres (6,600 ft), the highest peak being Doddabetta, at 2,637 metres (8,652 ft).