The relationship between boreal summer intraseasonal oscillation (BSISO) and precipitation extremes over Indonesia is investigated using observational datasets from 30 years (1987-2016) of rain gauge measurements and the gridded Asian Precipitation-Highly Resolved Observational Data Integration Toward Evaluation of Water Resources (APHRODITE) from 1998-2015. The results indicate that the frequency of extreme precipitation events in Indonesia (defined as total precipitation above the 95 th percentile) during extended boreal summer (May-August) is significantly modulated by BSISO, especially over the western and northern regions. Under the influences of BSISO1, the probability of the precipitation extremes over Sumatra and Borneo increases by 20-120% during phases 1 to 3, and approximately 50-80% over the eastern part of Borneo and Sulawesi during phase 4. Under the BSISO2, the probability of the extremes increases up to 40% over Sumatra during phases 1 to 2 and up to 140% over the Borneo and Sulawesi during phases 2-3. The increase in the probability of extreme summer precipitation is associated with enhanced large-scale moisture flux convergence and upward moisture transport induced by BSISO active phases. These results may provide valuable information for medium-to-extended-range prediction of summer precipitation extremes in Indonesia.