Dr David Duncan Main (1856-1934) was a British doctor, best known for his medical missionary work in Hangzhou, the capital of the south-eastern Chinese Province Zhejiang, during the late 1800s and early 1900s. He was superintendent of the Church Missionary Society (CMS) Medical Mission from his arrival in 1881 until 1927, and principal of the Hangzhou Medical Training College. He was also involved in the formation of the Young Men's Christian Association (YMCA) in Hangzhou, which focused on preaching the Gospel to non-Christians. Main and his wife, Florence Nightingale Smith, opened over thirty medical buildings during their time in China, as well as a centre for industrial work training.