Theni District (Tamil: தேனி மாவட்டம்) is a district of Tamil Nadu state in South India. The city of Theni is the district headquarters. The district is divided into two natural divisions: The hilly areas constituted by parts of the three taluks Periyakulam, Uthamapalayam and Andipatti with thick vegetation and perennial streams from the hills on the western side and Cumbum valley which lies in Uthamapalayam taluk.

Theni District was formed by the bifurcation from erstwhile Madurai District as per G.O. Ms. No. 679 Revenue Department Dated July 7, 1996. Consequent to the bifurcation, one new Revenue Division with headquarters at Uthamapalayam and two new Taluks at Theni and Bodinayakanur were also created with effect from January 1, 1997. Theni Municipal town was only a firka headquarters until December 31, 1996. Consequent to the formation of the new District, Theni Municipal Town was upgraded as the Taluk and District headquarters on January 1, 1997.

The region covered by present day Theni District was scarcely populated before 1900s. In 1886 Mullaperiyar dam project brings some part of water from Periyaar River downhill to Cumbum Valley and merge into Mullai river. This project helped more people to settle down in Cumbum Valley. Also to be noted that Theni is itself a small and less-known town by 1900s. After the Mulllai-Periyaar merger project lots of people from nearby dry arid regions (Sivakasi, Kovilpatti, Virudhunagar, Sattur and several nearby towns) settled in Cumbum-Valley (present day Theni District). So there was influx of people by 1890s to 1920s. Bodi and Allinagaram are famous places at that age. Later Theni developed in rapid pace due to lots of business activities.

Theni pronunciation (help·info) is a town in the Western Tamil Nadu state of South India. It is the headquarters of Theni District. It is known for the large-scale trading of Cardamom, Cotton, Grapes, Chilli and Textiles. It hosts the second largest weekly market in Tamil Nadu and the fourth largest in South India. It is well-connected by rail and road to Madurai and by road to all the important cities in Tamil Nadu. The nearest airport is at Madurai.

Theni district is surrounded by the Western Ghats. The main occupation of people of Theni is agriculture. The Vaigai River flows through this district. Other important rivers include the Mullaiperiyar, Suruliyaru, Varaganadi, Kottakkudiyaru, and Manjalaru rivers.

Theni has a historic background during the 19th century. There were many antagonists against British rule like Marutha Nayagam.

The famous Saneeswaran (God of Saturn) temple is located in the Kucchanur Village.It is a Suyambhu statue and Pathirakaliyamman Temple, Pathirakalipuram.

Theni district is rich in flora and fauna. Many dams have been constructed on the rivers, including Vaigai Dam, Sothupparai Dam, Manjalaru Dam, and Meghamalai Dam. Several famous waterfalls are here: Suruli Falls, Kumbakarai Falls and Meghamalai Falls. Theni is surrounded by many tourist attractions like Kodaikannal, Thekkady, Meghamalai, Munnar, Kumbakkarai, Sothuppaarai and Vaigai dam. Because Theni's scenic environment, it is one of the most favoured shooting spots of the Tamil film industry. Thekkady wildlife sanctuary is a nearby attraction which lays in the border between Kerala and Tamil Nadu. There is a temple where Kannaki (historic character) was believed to be turned statue.