Haimen (, Qihai dialect: , Shanghai: ) is a county-level city under the administration of Nantong, Jiangsu province, with a population of approximately 1 million. It is located at the opposite side of the Yangtze River to Shanghai and is directly north of Chongming Island except for a small portion that forms Haimen City's Haiyong Township. Chonghai Bridge links the city to Chongming County. Haimen is the seat of the Roman Catholic Diocese of Haimen.