Premenstrual dysphoric disorder (PMDD) is a severe form of premenstrual syndrome, characterized by depressive, anxious, and somatic symptoms, associated to a significant reduction in social functioning and quality of life. Although selective serotonin reuptake inhibitors (SSRIs) and oral contraceptives show a substantial effectiveness in PMDD, a wide percentage of women affected by PMDD do not respond to those first-line treatments. We present the case of a 42-year-old woman, affected by a disabling form of PMDD, who partially responded to an SSRI treatment and significantly improved after an add-on therapy with lamotrigine. We therefore suggest that lamotrigine may be an efficacious and safe augmentation strategy in severe PMDD cases showing unsatisfactory or partial response to SSRI treatments.