China started market-oriented reforms in the late 1970s, and a fundamental social change in post-reform China has been massive migration. The primary purpose of this research is to examine the effect of migrant inflows on online expressions of regional prejudice, which is defined as expressions of antipathy toward migrants. In particular, we examine the dynamics of expressions of regional prejudice on Weibo, a popular Twitter-like social media site in China. Furthermore, we explain online expressions of regional prejudice through the lens of the group-threat and contact theories, and seek potential conditions that facilitate the development of a positive relationship between local residents and migrants. Our findings lend strong support to the group-contact argument that migrant inflows reduce online expressions of regional prejudice in China. The contact effect is larger when migrant inflows are measured in terms of migrant labor and interprovince migrants, who represent China’s typical migrants. Furthermore, the contact effect is only observed in relatively small cities, while a small threat effect is observed in mega and super-mega cities where the population-control system is strictly maintained.