The CSI Tiruchirappalli Thanjavur Diocese is a diocese of Church of South India in Tamil Nadu state of India. The diocese is one among the 24 dioceses of Church of South India and was one of the first 14 dioceses to be formed at the inaugural of the Church of South India in 1947.

Tiruchirappalli, also called Tiruchi or Trichy, is a major tier II city in the Indian state of Tamil Nadu and the administrative headquarters of Tiruchirappalli district. The city is credited with being the best livable city, the cleanest city of Tamil Nadu, as well as the fifth safest city for women in India. It is the fourth largest city as well as the fourth largest urban agglomeration in the state. Located 322 kilometres (200 mi) south of Chennai and 374 kilometres (232 mi) north of Kanyakumari, Tiruchirappalli sits almost at the geographic centre of Tamil Nadu state. The Cauvery Delta begins 16 kilometres (9.9 mi) west of the city where the Kaveri river splits into two, forming the island of Srirangam which is now incorporated into the Tiruchirappalli City Municipal Corporation. The city occupies an area of 167.23 square kilometres (64.57 sq mi) and had a population of 916,857 in 2011.