This study aims to analyze the impact of fiscal decentralization on development performance in West Kalimantan Province with opportunistic behavior as a mediating variable. The background of this research is the importance of fiscal decentralization as a policy instrument to encourage regional development. However, there are concerns that opportunistic behavior in regional financial management can reduce the effectiveness of fiscal decentralization. The research method used is panel data analysis that combines time series and cross-section data from 14 districts/cities in West Kalimantan during the 2017-2022 period. The results of the study show that budget allocation does not have a significant influence on opportunistic behavior but has a significant positive effect on economic performance, while transfer funds actually increase fiscal opportunistic behavior and significantly negatively affect economic performance. In addition, opportunistic fiscal behavior has been proven to reduce regional economic performance. These findings underscore the importance of better fiscal management to minimize opportunistic behavior and maximize the benefits of fiscal decentralization.