The Church of South India is a member of the Anglican Communion and its bishops participate in the Lambeth Conferences; it has representation on the Anglican Consultative Council. It is also a member of the World Council of Churches, the World Alliance of Reformed Churches, and the National Council of Churches in India. The CSI is in full communion with the Malankara Mar Thoma Syrian Church of India and the Church of North India (CNI). Following the formation of the Church of North India in 1970, these three churches which have bishops as their heads, began discussions on closer cooperation with the possibility of full union at a later date. A Joint Theological Commission (JTC) established for this purpose at its first meeting in Madras in 1975 defined its objective as “To explore the possibilities of closer cooperation between the three churches and to discuss questions of faith and order and other relevant issues so that they may be led to eventual Union keeping in mind the ultimate goal of all Christ’s people in India for the fulfillment of the Mission of the Church.

The Church of South India (CSI), Church of North India (CNI), and Malankara Mar Thoma Syrian Church of India jointly formed the Communion of Churches in India (CCI) in 1978 for mutual recognition of the ministry and leaders, inter communal relationship, and to explore possibilities of working together especially in the field of evangelization in India and other areas of cooperation in the fulfillment of the mission of the Church. Later, the Methodist Church in India is also joined the Communion. The Church of South India maintains a partnership with the Church of North India, Church of Scotland, Episcopal Church, Mar Thoma Syrian Church of Malabar, Methodist Church of Great Britain, Presbyterian Church in Korea, Presbyterian Church in the Republic of Korea, Presbyterian Church of India, Presbyterian Church (USA), Reformed Church in America, United Church of Christ, and the Uniting Church in Australia.