A large Swedish nationwide cohort study of more than 3.6 million women found a strong bidirectional association between premenstrual disorders (PMD) and psychiatric disorders. Women with PMD had about double the risk of later developing a psychiatric disorder, while women with psychiatric disorders were similarly more likely to later be diagnosed with PMD. The strongest associations were seen with depression, anxiety, ADHD, bipolar disorder, and personality disorders, and these relationships persisted even after accounting for shared family and genetic factors through sibling analyses. The findings suggest that PMD and psychiatric disorders may share underlying biological and genetic mechanisms and support the need for menstrual cycle–informed psychiatric care.