<p>Based on the hourly rain gauge data from 86 stations in Guangdong during the flood season (April to September) from 2008 to 2021, the diurnal variations in rainfall with different durations from coastal to inland areas are studied. The results indicate that short-duration rainfall initiates and peaks in the afternoon, with a large diurnal amplitude. Long-duration rainfall events tend to initiate during the late night (22–06 LST (local solar time)) in coastal and coastal–inland transitional regions, whereas in inland regions, they often initiate in the afternoon (13–20 LST) or late night (23–04 LST). The long-duration rainfall peaks in the morning (05–10 LST), with differences in the timing of the rainfall peak in different regions. The diurnal amplitude of long-duration rainfall is large in coastal areas and decreases toward inland areas. The diurnal variation in long-duration rainfall is determined mainly by the diurnal variation in rainfall intensity, and the peaks of heavy and weak long-duration rainfall in coastal regions, both of which occur around the early morning, are consistent with the total rainfall. In contrast, the diurnal variation in transitional and inland areas is a factor of the morning peak value of heavy long-duration rainfall and the afternoon peak value of weak long-duration rainfall. Additionally, the regional differences in the peak time of heavy long-duration rainfall are influenced by regional differences in both the start time and the time from the start to the peak of heavy long-duration rainfall. Finally, a preliminary exploration is conducted to determine the possible reasons for the initiation and peak of heavy long-duration rainfall.</p>