ObjectiveTo assess the impact of COVID‐19 on trends in postpartum mental health diagnoses and utilization of psychotherapy and prescription drug treatment.Data SourcesData were obtained from a large, national health insurance claims database that tracks individuals longitudinally.Study DesignWe used interrupted time series models to examine changes in trends of postpartum mental health diagnoses before and during the COVID‐19 pandemic and t‐tests to examine differences in treatment.Data Extraction MethodsWe used billing codes to identify individuals who received mental health‐related diagnoses and treatment in the first 90 days after a birth hospitalization. We excluded individuals diagnosed with schizophrenia or bipolar disorder and those with an unknown payer at delivery.Principal FindingsCompared to the pre‐pandemic period, the trend in new postpartum mental health diagnoses increased significantly in the post‐COVID‐19 period (0.06 percentage points [95%CI 0.01, 0.11]). Over 12 months, the percentage of new diagnoses was 5.0% greater relative to what would be expected in absence of COVID‐19. The percentage of diagnosed individuals who did not receive treatment increased from 50.4% to 52.7% (p = 0.003).ConclusionsFindings point to an urgent need to improve screening and treatment pathways for perinatal individuals in the wake of COVID‐19.