The death of a loved one is associated with an increased risk of developing severe mental health complications such as prolonged grief disorder (PGD), post-traumatic stress disorder (PTSD) and depression. Research supports the efficacy of cognitive-behavioural therapies (CBT), delivered online or face-to-face, in treating post-loss mental health problems. However, evidence-based treatments for bereaved adults remain scarce within the Swedish healthcare system. The main aim of this study was to evaluate the feasibility and acceptability of grief-focused internet-delivered CBT (ICBT) for PGD in Swedish bereaved adults, using an uncontrolled multiple-methods design. A secondary aim was to assess the potential effect on symptoms of PGD, PTSD and depression. Fourteen participants who met criteria for probable PGD were enrolled in the study and received access to the online treatment for eight weeks. Ten participants completed post-treatment interviews and self-report questionnaires about their experiences of the treatment. Self-reported symptom-levels of PGD, PTSD and depression were assessed at baseline, mid-, posttreatment and at four-month follow-up. Overall, the grief-focused ICBT was perceived as helpful and emotionally engaging. Most participants reported satisfaction with the intervention. Reductions were observed in symptoms of PGD, PTSD and depression, although these findings should be interpreted as preliminary due to the small sample size and limited statistical power. Overall, the results suggest that grief-focused ICBT is a feasible and acceptable treatment for bereaved adults. A fully powered randomised controlled trial is warranted to establish the efficacy of the treatment.