Premenstrual dysphoric disorder (PMDD) is a debilitating cyclic mental disorder affecting about 2-5% of women of reproductive age. PMDD is characterised by recurring emotional, behavioural, cognitive, and somatic symptoms that arise during the luteal (premenstrual) phase of the menstrual cycle and remit shortly after the onset of menses. Although pharmacological interventions are available, many women experience residual symptoms, discontinue treatment or refrain from them because of side effects. Therefore, non-pharmacological treatment options are needed. Preliminary evidence suggests that internet-delivered cognitive behavioural therapy (ICBT) is a promising candidate, but further research is warranted. Also, there is room for treatment improvement. Specifically, it has been suggested that components targeting emotional and interpersonal dysregulation should be incorporated into CBT for PMDD. The current study aims to assess the effects of an ICBT intervention for PMDD incorporating skills training in emotion regulation and interpersonal effectiveness in a randomised controlled trial (RCT).