Evaluations of evidence-based, easily accessible, psychological interventions to improve maternal mental health following very preterm birth are scarce. This study investigated the efficacy and acceptability of the expressive writing paradigm for mothers of very preterm infants. The level of maternal posttraumatic stress and depressive symptoms was the primary outcome. Participants were 67 mothers of very preterm babies who were randomly allocated into the intervention (expressive writing; n = 33) or control group (treatment-as-usual; n = 32) when their infant was aged 3 months (corrected age, CA). Measurements were taken at 3 months (pre-intervention), 4 months (post-intervention), and 6 months CA (follow-up). Results showed reduced maternal posttraumatic stress (d = 0.42), depressive symptoms (d = 0.67), and an improved mental health status (d = 1.20) in the intervention group, which were maintained at follow-up. Expressive writing is a brief, cost-effective, and acceptable therapeutic approach that could be offered as part of the NICU care.