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Background. Men who have sex with men (MSM) are highly vulnerable to human immunodeficiency virus (HIV) infection and more likely to migrate due to widespread stigma and discrimination in China. Their mobility complicates estimation of local MSM population sizes and the provision of HIV services, and may also contribute to the spread of HIV.Methods. Between 1 January 2008 and 31 December 2012, the visits of all individuals to the largest Chinese MSM dating website were recorded. After a predesigned de-identification procedure by the website, we analyzed Internet Protocol addresses for migration patterns. Migrants were defined as individuals who were away from their registered residence for >6 months in the last 12 months.Results. The website contained data on 794 912 MSM eligible for the study, of which 34.5% were migrants. The median age was 26 years (range, 18–61 years), and 85.5% were unmarried. Compared with nonmigrant MSM, migrants were less likely to be married to a woman (8.6% vs 13.5%; P < .001). The 5 provinces with the highest migrant inflow ratios were Guangdong, Shanghai, Beijing, Tianjin, and Zhejiang. Eastern coastal cities were the primary destination of MSM from southwestern China.Conclusions. Preferential MSM migration may influence MSM population sizes in both originating and destination provinces, particularly for provinces with uneven inflow and outflow. MSM migration from southwestern China, which has the highest HIV prevalence in this population, to coastal cities with lower prevalence may have implications for the spread of the HIV epidemic as well as HIV care services.