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The summer precipitation over the southeastern Tibetan Plateau (SETP) exhibits significant diurnal variability, with a notable peak before midnight. This study investigates how thermally forced circulations regulate the diurnal precipitation over the SETP, using surface rain‐gauge observations and high‐resolution reanalysis data from June to August during 2015–2019. Water vapor supply and precipitation onset in the afternoon are associated with the upslope winds and upward motions of the mountain–plains solenoid (MPS) between the SETP and nearby lowlands (the Sichuan Basin and Yunnan–Guizhou Plateau). After sunset, the sustained easterly and stronger southerly winds below 500 hPa transport abundant moisture from the rainy lowlands to the SETP. The findings suggest that these winds are mainly due to the larger zonal continental‐scale thermally forced circulation combined with the meridional large‐scale MPS before midnight and the meridional small‐scale MPS after midnight. Plain Language Summary: The summer rainfall over the southeastern Tibetan Plateau (SETP) is crucial to the hydrological cycle of both the SETP and nearby regions. However, it remains unclear why the rainfall over the SETP mostly occurs at night due to limited observations. This study aims to investigate the role of thermally driven winds in diurnal rainfall during summer (June–August) over the SETP, using high‐resolution reanalysis data and surface rain‐gauge observations. The results show that the strong upward motions and low‐level upslope winds of the thermally forced mountain‐valley wind systems (MVWS) between the SETP and the lowlands (Sichuan Basin and Yunnan–Guizhou Plateau) help to trigger the precipitation and transport moisture in the afternoon. After sunset, thermally forced circulations, including the east–west continental‐scale thermal circulation between the Asian continent and its adjacent ocean, the north–south large‐scale MVWS between the Yunnan–Guizhou Plateau and its southern plain, and the north–south small‐scale MVWS between the SETP and its southern slope, help maintain horizontal winds from the rainy lowlands, transporting abundant moisture to facilitate the rainfall over the SETP at night. Key Points: Thermally forced circulations greatly impact the diurnal cycle of summer precipitation over the southeastern Tibetan Plateau (SETP)Daytime rainfall over this region is associated with the upward motions and upslope winds of the mountain–plains solenoidMulti‐scale thermal circulations are responsible for moisture transport of nighttime rainfall over the SETP [ABSTRACT FROM AUTHOR]