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Urban land expansion is one of the most conspicuous aspects of urbanization and has profound impacts on regional climate change. Most studies, however, focus on its impacts on surface temperature, and possible effects on atmospheric humidity are less known. By examining the humidity changes in the Yangtze River Delta urban agglomeration of China during 1961-2014, we find a prominent urban dry island (UDI) effect in this urban agglomeration, as characterized by reduced humidity and increased vapor pressure deficit in the urban core area. In the past decades, the UDI effect has been significantly intensified by rapid urban land expansion. Urban expansion contributes to around half of the decrease in atmospheric humidity and the increase in vapor pressure deficit in urban areas. These effects are particularly stronger in wet hot summer and relatively weaker in cold dry winter. We suggest that the UDI effect should be considered in future urban planning, landscape design, and climate change assessment and mitigation.