The need for clean water in mosques, especially for ablution water needs, is something that needs to be addressed. Lack of clean water during the dry season can be a problem for some mosques, such as the Al-Mu'minin mosque which experiences a shortage of groundwater volume during the dry season and groundwater becomes a little smelly. This study aims to determine the potential of rainwater through hydrological analysis and can plan a rainwater harvesting system at the Al-Mu'minin mosque. The results obtained are the amount of water needed for ablution at the Al-Mu'minin mosque is as much as 6.97 m3/day or equivalent to 209.11 m3/month. This result is obtained from the calculation of the number of worshipers per day with the need for ablution water per prayer time, which is 4.42 liters/person. Next, a hydrological analysis was calculated and the result was 45.51 mm/hour of rain intensity, which was then used to calculate the volume of rainwater that could be accommodated so that the result was 7.81 m3/day or equivalent to 234.3 m3/month. Based on the calculation results, the piping system used is 4 inches for horizontal pipes and 3 inches for standpipes.