In bereavement, some individuals develop complicated grief reactions (CGR), including symptoms of prolonged grief disorder (PGD), posttraumatic stress disorder (PTSD), and depression. Symptoms of PGD often co-occur with other complicated grief reactions, e.g., PTSD and depression, pointing to the need for a transdiagnostic understanding of CGR to inform treatment. In this paper, a transdiagnostic cognitive-behavioral therapy for CGR called “CBTgrief” is explained, including its theoretical framework and treatment content.