Mixed halide perovskites exhibit great potential as materials for the future generation of photovoltaic devices. Yet, their reaction to moisture remains uncertain, necessitating further exploration. While prolonged exposure to moisture can lead to degradation, it can also passivate traps at an optimal moisture level. Here, we use scanning probe microscopy to perform nanoscale moisture-dependent photovoltaic characterizations of open and compressed grain boundary (GB) structures of wide bandgap (FAPbI₃)_0.3(FAPbBr₃)_0.7 perovskites. The investigation reveals a decrease in the potential barrier at compact GBs with increasing moisture levels, contrasting with the behavior observed in open GBs. Moreover, the photocurrent distribution over both samples proportionally increases when relative humidity (RH) is raised from 10% to 60%. Notably, following a 24-h exposure at RH 60%, the compact-GB sample demonstrates: ⅰ) a reduction in the density of charged trap states at GBs, ⅱ) higher photocurrent, accompanied by a noticeable decrease in current hysteresis compared to the open GB sample, and ⅲ) further enhancement in device efficiency and crystallinity compared to devices with open GBs. These findings suggest that optimizing humidity conditions in engineering the GB chemistry can enhance the optoelectrical properties of GBs, ultimately leading to improved device performance.