This study proposed a new drought index considering the accumulative pattern of daily rainfall, i.e., Rainfall Accumulation Drought Index (RADI). The RADI can be easily calculated at daily scale by comparing the long-term averaged cumulative rainfall to the observed cumulative rainfall for a specific duration. This study evaluated the availability of the RADI in the field of monitoring short-term and long-term droughts by investigating the spatial and temporal variability and the recurrence cycle of drought in South Korea. To present the short-term and long-term droughts, the various SPIs with different durations should be used in practice. However, the RADI can present and monitor both short-term and long-term droughts as a single index. By investigating the national average of the RADI, specific drought patterns of 20-year cycle were identified in this study. This study also proposed a five-level drought classification considering occurrence probability that would be a suitable alternative as a drought criterion for drought forecast/response.