OBJECTIVES:  Postpartum depression (PPD) risk has been reported to be correlated with socioeconomic factors. More than 50% of US deliveries are covered by Medicaid, for which all pregnant women with an income <133% of the federal poverty line are eligible. Although Medicaid covers a large portion of deliveries, few estimates of PPD prevalence have examined this population. The objective of this study was to estimate PPD prevalence in the Medicaid population using a large population database. METHODS:  This retrospective analysis used the Truven Medicaid database (>30 million patients; 11 states) to identify claims for women who were 15-50 years old at the time of delivery from 2012-2014 based on ICD-9 coding. PPD cases were identified based on a combination of inpatient and outpatient claims for PPD or major depression (MDD), adjustment disorder or depression not otherwise specified, or claims for treatment. Women with continuing MDD were excluded. Prevalence and disease severity were estimated. RESULTS:  In this Medicaid database population, the algorithm determined a PPD prevalence of 1.7%, 1.9%, and 2.1%, for the years 2012-2014, respectively. In 2014, no difference was observed in PPD prevalence in women ≤17 years old (2.2%) and in either women 18-34 or 35 or older (both 2.1%). Prevalence was also estimated by race/ethnicity, with rates of 2.8% among Whites, 1.2% among Blacks, 1.3% among Hispanics, and 1.9% among others. Overall, mild cases accounted for 6.7% of the population, moderate 88.5%, and severe 4.8%. CONCLUSIONS:  PPD prevalence estimates were determined by the algorithm in a low-income population insured by Medicaid. Previous literature indicated that lower income and socioeconomic status may increase risk for PPD. However, our findings reveal a lower prevalence within this large US Medicaid population database than cited in the literature, which suggests an estimated prevalence of 10-20%, suggesting that PPD may be markedly underdiagnosed.