Mangaladevi Kannagi Temple is the oldest temple in India built around 1st century AD and is located in present day border of Theni District of Tamil Nadu and Idukki District of Kerala, about 7 km from Pazhiyankudi in Theni district and 15 km from Thekkady in Idukki.

Kannagi is an icon hailed in Tamil Epic Cilappatikaram. Cheran Chenguttuvan, the Tamil king of ancient Chera nadu, had erected the temple for Kannagi around 2000 years back during the Sangam period at Vannathipara and called it "Kannagi Kottam" or Mangaladevi Kannagi temple and performed regular pujas. Entry to the temple is allowed only on one day of the year; Chitra Poornami day. As it is a disputed area, entry to the temple is only in the presence of Theni and Idukki district collectors and police chiefs.

It seems that the temple originally belonged to Theni district as per States reorganization but annexed by Kerala government gradually. Today the temple fully falls within Idukki district border though the dispute still continues. The Tamil Nadu government has shown intention to retrieve the temple back to their jurisdiction.