Dr. Koyama has published some thirteen books, including a three volume work titled On Christian Life (available only in Japanese), and one hundred scholarly articles. Perhaps his best known works in English are Waterbuffalo Theology (1974) and Mt. Fuji and Mt. Sinai (1984).
His ecumenical experiences include being active in the Christian Conference of Asia under whose aegis he was a participant in the Asian Missionary Consultation, the Conservative-Ecumenical Consultation in Asia, the Dialogue with Buddhists, and the Theological Education Consultation. In the World Council of Churches he has participated in Faith and Order, Humanum Studies, the Dialogue with People of Other Faiths and Ideologies, Church and Society, the Nairobi Assembly, and the commission on World Mission and Evangelism.
He has also served as Assembly Preacher for the 190th General Assembly of the United Presbyterian Church in the USA , and for the Sweden "Kirchentag", was the keynote speaker at the 75th Anniversary of the 1910 International Missionary Conference, Edinburgh, and has lectured at the Biennial Assembly of the Association of Theological Schools in the United States and Canada, the International Association for Mission Studies, the European Gathering of Congregational Groups, Teologiska Seminariet, the Kanuga Bowen Conference, and delivered the Second Bonhoeffer Lecture in Berlin.