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During boreal summer, the patterns of precipitation anomalies in East Asia have notable month‐to‐month variations, which may induce alternated wet and dry conditions in different regions of East Asia during summer. In this study, based on observation and reanalysis data, the dynamic processes of monthly precipitation anomalies in three sub‐regions of East Asia (South China, the Yangtze River valley and northern–northeastern China) are investigated, which are associated with different combined effects of tropical air–sea interaction and mid‐to‐high latitude Rossby waves during different months. These dynamic processes dominate the leading modes of month‐to‐month variation of precipitation anomalies in East Asia during summer. To understand what dynamic processes are the key sources of the predictability of month‐to‐month variation of precipitation anomalies in East Asia during summer, a method of spatially running pattern correlation coefficient (SR_PCC) is applied to examine the differences in prediction skill of geopotential height anomalies, sea‐level pressure anomalies and sea surface temperature anomalies between the years of well and poorly predicted precipitation anomalies. Based on the results of CFSv2 hindcast and CMIP6‐AMIP experiments, both the air–sea interaction over tropical Indo–Pacific region and the mid‐to‐high latitude Rossby waves over the Eurasian continent are key sources of predictability of month‐to‐month variation of precipitation anomalies in East Asia during boreal summer. [ABSTRACT FROM AUTHOR]