Introduction Grief is an emotional reaction to the loss of a loved one with a natural recovery. Approximately 10% of people who lose a loved one develop prolonged grief disorder (PGD). Internet-based and computer-based interventions (ie, internet-delivered cognitive-behavioural therapy, iCBT) are a cost-effective alternative that makes it possible to reach more people with PGD. The main aim of this study is to assess the feasibility of a new iCBT¿called GROw¿for PGD. As a secondary objective, the potential effectiveness of GROw will be explored. Methods and analysis This study is a two-arm feasibility randomised trial. A total of 48 adults with PGD who meet the eligibility criteria will be randomised to the experimental group (iCBT: GROw) or the active control group (face-to- face CBT treatment). The treatment is organised sequentially in eight modules in the iCBT format and 8-10 sessions in the face-to- face format, and both formats have the same therapeutic components. There will be five assessment points with qualitative and quantitative evaluations: screening, baseline, after the intervention, 3-month follow-up and 12-month follow-up. Consistent with the objectives, the measures are related to the feasibility outcomes for the main aim of the study (participant adherence, expectations and satisfaction with the treatment, preferences, alliance and utility) and psychological and mental health outcomes for secondary analyses (symptoms of grief, symptoms of depression, symptoms of anxiety, affectivity, quality of life, work and social adaptation, post-traumatic growth, purpose in life, mindfulness and compassion). Ethics and dissemination The Ethics Committee of the Universitat Jaume I (Castellón, Spain) granted approval for the study (CD/002/2019). Dissemination will include publications and presentations at national and international conferences. Trial registration number NCT04462146