Women suffering from first onset postpartum mental disorders (PPMD) have a highly elevated risk of suicide. The current study aimed to: (1) describe the risk of self-harm among women with PPMD and (2) investigate the extent to which self-harm is associated with later suicide. We conducted a register-based cohort study linking national Danish registers. This identified women with any recorded first inpatient or outpatient contact to a psychiatric facility within 90 days after giving birth to their first child. The main outcome of interest was defined as the first hospital-registered episode of self-harm. Our cohort consisted of 1 202 292 women representing 24 053 543 person-years at risk. Among 1554 women with severe first onset PPMD, 64 had a first-ever hospital record of self-harm. Women with PPMD had a hazard ratio (HR) for self-harm of 6.2 (95% CI 4.9-8.0), compared to mothers without mental disorders; but self-harm risk was lower in PPMD women compared to mothers with non-PPMD [HR: 10.1, (95% CI 9.6-10.5)] and childless women with mental disorders [HR: 9.3 (95% CI 8.9-9.7)]. Women with PPMD and records of self-harm had a significantly greater risk for later suicide compared with all other groups of women in the cohort. Women with PPMD had a high risk of self-harm, although lower than risks observed in other psychiatric patients. However, PPMD women who had self-harmed constituted a vulnerable group at significantly increased risk of later suicide.