The Diocese of Quilon is a Latin Church ecclesiastical jurisdiction or diocese of the Catholic Church based in the southern Indian city of Kollam. It is a suffragan in the ecclesiastical province of the metropolitan Archdiocese of Trivandrum. The Diocese of Quilon covers an area of 1,950 km2 that contains a population of some 4.8 million. At least 4.8% of the people in the area are Catholic.

Kollam, also known by its former name Quilon, is an ancient seaport and city on the Malabar Coast of India bordering the Laccadive Sea, which is a part of the Arabian Sea. It is 71 km (44 mi) north of the state capital Thiruvananthapuram. The city is on the banks of Ashtamudi Lake and the Kallada river. Kollam is the fourth largest city in Kerala and is known for cashew processing and coir manufacturing. It is the southern gateway to the Backwaters of Kerala and is a prominent tourist destination. Kollam is one of the most historic cities with continuous settlements in India. Geographically, Quilon formation seen around coastal cliffs of Ashtamudi Lake, represent sediments laid down in the Kerala basin that existed during Mio-Pliocene times.