Abstract: This study focuses on the interannual variation of monthly precipitation in summer (i.e., June–September) over the Indochina Peninsula from 1979 to 2019 and confirms that the precipitation in different months is independent of each other. Although the summer monthly precipitation is generally affected by the easterly anomalies from the tropical Pacific Ocean and the westerly anomalies from the Indian Ocean, the regulatory factors causing these circulation anomalies are different. The results show that the tropical West Indian Ocean sea surface temperature (SST) anomalies largely modulate the precipitation in June over the Indochina Peninsula. They regulate the vertical movement near the Indochina Peninsula by influencing the convergence and divergence circulation and result in precipitation anomalies in the Indochina Peninsula. In July, the precipitation over the Indochina Peninsula is closely related to the SST anomalies in the tropical central and eastern Pacific region in the previous winter, which regulates the precipitation by influencing the northwest Pacific subtropical high activities. In addition, the precipitation in August is mainly affected by the simultaneous east–west dipole SST anomalies around the tropical western Pacific and Maritime Continent. These SST anomalies can regulate the precipitation in the Indochina Peninsula through local Hadley circulation anomalies and Rossby waves. Relatively, there was no large-scale tropical SST signal related to precipitation in the Indochina Peninsula in September that was notably significant. However, the region is significantly affected by typhoon activities, particularly along its eastern coastal areas. This paper helps further understand the interannual variation of summer monthly precipitation and provides a scientific reference for the summer precipitation climate forecast in the Indochina Peninsula.