Postpartum depression affects approximately 1 in every 8 postpartum individuals in the US. Antidepressant medication can effectively treat postpartum depression. However, gaps in postpartum insurance coverage after the end of Medicaid pregnancy coverage at 60 days postpartum may limit treatment uptake and decrease continuity of postpartum depression treatment. To examine the association of Medicaid expansion in Arkansas with postpartum antidepressant prescription fills and antidepressant continuation and supply during the first 6 months postpartum. Cohort study with a difference-in-differences analysis comparing persons with Medicaid and commercially financed childbirth using Arkansas' All-Payer Claims Database (2013-2016). Analysis was completed between July 2021 and June 2022. Medicaid-paid childbirth after January 1, 2014. Antidepressant medication prescription fills and the number of days of antidepressant supply in the early (first 60 days after childbirth) and the late (61 days to 6 months after childbirth) postpartum periods. In this cohort study with a difference-in-differences analysis of 60 990 childbirths (mean [SD] birthing parent's age, 27 [5.3] years; 22% Black, 7% Hispanic, 67% White individuals), 72% of births were paid for by Medicaid and 28% were paid for by a commercial payer. Before expansion, 4.2% of people with a Medicaid-paid birth filled an antidepressant prescription in the later postpartum period. Medicaid expansion was associated with a 4.6 percentage point (95% CI, 2.9-6.3) increase in the likelihood, or a relative change of 110%, in this outcome. Before expansion, among people with postpartum depression in the early postpartum period with a Medicaid-paid birth, 32.7% filled an antidepressant prescription in the later postpartum period, and had an average of 23 days of antidepressant prescription supply during the later postpartum period. Among people with early postpartum depression, Medicaid expansion increased the continuity of antidepressant treatment by 20.5 percentage points (95% CI, 14.1-26.9) and the number of days with antidepressant supply in the later postpartum period by 14.1 days (95% CI, 7.2-20.9). Medicaid expansion in Arkansas was associated with an increase in postpartum antidepressant prescription fills, and an increase in antidepressant treatment continuity and medication supply in the period after Medicaid pregnancy-related eligibility ended.