Objective <p>This study aimed to investigate the relationship between domestic water hardness and risk of symptomatic abdominal aortic aneurysm (AAA) and explore whether this association was modified by genetic predisposition to AAA.</p> Methods <p>Exposures of interest included concentration of calcium (Ca), magnesium (Mg), calcium carbonate (CaCO₃) as well as water hardness. Symptomatic AAA was identified through ICD code and operation code. Multivariable Cox proportional-hazard models were used for outcome analysis. Subgroup analysis was performed by the levels of genetic susceptibility to AAA.</p> Results <p>This study included 371,668 participants. Over a median follow-up duration of 15.2 years, 2,154 new cases of symptomatic AAA were recorded. A negative association was observed between exposure to Ca and CaCO₃ and water hardness and risk of symptomatic AAA (all <i>p values &lt; 0.050</i>). No significant interaction was observed between genetic risk, as determined by AAA polygenic risk score (PRS), and any water mineral exposure in relation to AAA risk.</p> Conclusion <p>Our study found that exposure to domestic hard water was associated with a reduced risk of symptomatic AAA, regardless of individuals ' genetic risk. These findings reinforce the recommendations on the importance of maintaining and supplementing minerals in drinking water, such as using filtered or desalinated water.</p>