Rainfall is one of the crucial types of data for hydrometeorological studies. In the context of climate change, which has become increasingly complex, hydrometeorological forecasting data are urgently necessary for further climatic simulation modeling. However, it is always a big challenge to achieve accurate rainfall data for such a minimum observation cost. Hence, an optimization design for the rain-gauge network in southern Vietnam was developed in this study, with a focus on the quantity and spatial distribution of rain gauges. The well-known geostatistics method combined with a spatial interpolation algorithm was applied using input rainfall data over the past 40 years. The selection of the appropriate spatial interpolation method affected the accuracy of the optimization process. The results of the interpolation analysis indicated the spatial variability of the rainfall patterns throughout the study area according to the seasons, as well as the geographic distribution of the rainfall, especially in the coastal area. The primary criterion for reducing the estimated data’ uncertainty is improving the network density by installing additional rain-gauge stations. In this research, after the optimal number of stations was determined, a geographic information system-based approach and a multilayer overlaying technique were used to obtain the optimal rain gauges spatial distribution map. For future research, the proposed method is capable to apply in other related works to obtain more accurate rainfall estimation data.