Background. Nearly 20% of women will be confronted with anxiety or depressive disorders during the perinatal period and this may lead to adverse outcomes for both mother and child. Cognitive behavioural therapy (CBT) is the psychological intervention with the most empirical support for the clinical management of anxiety and depressive disorders. Depression and anxiety frequently occur in women during the perinatal period, and there is growing evidence that internet-delivered CBT (iCBT) could be an acceptable and effective intervention. ThisWayUp iCBT Virtual Clinic in Australia has developed a program for postnatal anxiety and depression. As part of a Québec-Ontario collaboration, this research protocol aims to examine the acceptability and efficacy of a French-Canadian adaptation of the postnatal anxiety and depression program. Methods/Design. The investigators propose to conduct a hybrid type 1 randomized clinical trial and implementation study to replicate the findings of the trial conducted in Australia as well as explore barriers and facilitators to potential large-scale implementation. Treatment and control groups. a) adapted postnatal depression and anxiety iCBT program (3 lessons to complete in six weeks) added to treatment-as-usual (TAU); b) TAU. Participants will include women with possible postnatal depression or anxiety as per the Generalized Anxiety Disorder-7 (GAD-7) or the Edinburgh Postnatal Depression Scale (EPDS). The primary outcome measures will be the GAD-7 and the EPDS. Secondary outcome measures will comprise self-reported instruments to evaluate mother-child experience, postnatal depression, psychological distress, suicidality risk, quality of life, treatment satisfaction and service utilization. Statistical analysis. Statistical analysis will follow intent-to-treat principles. A mixed model regression approach will be used to account for between- and within-subject variations in the analysis of the longitudinal effects of iCBT compared to TAU intervention. Moderation analyses will be performed for three sets of moderators: clinical characteristics, previous treatments and sociodemographic characteristics. Additionally, treatment effect sizes will be calculated with Cohen's d. Discussion. The study will deliver important data of efficacy and acceptability to patients, clinicians, and decision-makers to inform the scaling-up of the postnatal iCBT intervention. The benefits to patients could include timely access to services, improved remission rates and health-related quality of life, better long-term outcomes; and to the healthcare system, policy and decision-makers: improved efficiency, optimization of resources, and this to meet the perinatal women and child needs equitably.