Neighbourhood socioeconomic status (NSES) has been identified as a determinant of mental health. In this study, we aimed to quantify how many cases of common mental health problems could be prevented by increasing NSES in the most socioeconomically deprived neighbourhoods in Rotterdam, the Netherlands, and how the increases in NSES would affect mental health inequalities. We used publicly available data for conducting a quantitative Health Impact Assessment (HIA) of two counterfactual policy scenarios. In Scenario 1, we set the NSES to the Rotterdam median score for all neighbourhoods which were below the Rotterdam median. In Scenario 2, we set the NSES score to the Dutch national average socioeconomic status score for the neighbourhoods that were below the national average. We estimated that Scenario 1 could prevent 5847 (95% CI, 2700-7999) or 10.7% of annual cases of common mental health problems, and Scenario 2 could prevent 10 713 (95% CI, 4875-14 799) or 19.6% of annual cases in Rotterdam while also reducing mental health inequalities between neighbourhoods in both scenarios. Given the substantial improvements in population mental health that enhanced neighbourhood socioeconomic conditions would bring, policy implementation is urgently needed.