Background <p>Unstable access to safe drinking water may harm mental health, yet its impact by sex remains unclear. We aimed to explore sex-specific associations between household water sources and depressive symptoms (DS) in Chinese middle-aged and older adults.</p> Methods <p> 5,868 participants aged ≥ 40 years were selected from China Family Panel Studies (2012–2020). We used time-varying Cox models to assess the association between household water sources and DS. Group-based trajectory models were employed to identify water-use trajectories, and logistic regression was used to examine the relationship between trajectory groups and post-follow-up DS. Meanwhile, we quantified the increase in DS burden attributable to the use of unstable water.</p> Results <p>There was a significant association between household water sources and DS, with notable differences between sexes. For females, relative to stable water, using unstable water was significantly associated with higher follow-up DS risk [hazard ratio (HR) = 1.260, 95% confidence interval (CI): 1.116, 1.423], and this risk increased with greater water instability (cellar-stored water: HR = 1.240, 95% CI: 1.091, 1.409; surface-exposed water: HR = 1.403, 95% CI: 1.062, 1.853). However, these associations were not observed in males. Meanwhile, we identified three water-use trajectories by sex: compared to the constant stable water group, constant unstable water was significantly linked to post-follow-up DS, whereas switching from unstable to stable water reduced this risk in females but not in males. In addition, 5.42% of DS burden in females was attributed to unstable water use.</p> Conclusions <p>Unstable household water sources were associated with an increased DS risk in females but not in males. Sustainable policies securing reliable water supply are needed to protect females’ mental health.</p>