Background: Losing a loved one during the COVID-19 pandemic is a potentially traumatic loss that may result in symptoms of persistent complex bereavement disorder (PCBD), posttraumatic stress disorder (PTSD), and depression. To date, grief-specific cognitive-behavioural therapy (CBT) has mostly been delivered through individual face-to-face formats, while studies have shown that online treatment also yields promising results. Offering treatment online is now more than ever relevant during the pan demic and may offer important benefits compared with face-to-face CBT, such as lower costs and higher accessibility. Our expectation is that grief-specific online CBT is effective in reducing PCBD, PTSD, and depression symptoms. Objective: Our aim is to evaluate the short-term and long-term effectiveness of grief-specific online CBT in reducing PCBD, PTSD, and depression symptom-levels for adults who lost a loved one during the COVID-19 pandemic. Method: This study consists of two parts. In part 1, a two-armed (unguided online CBT versus waitlist controls) randomized controlled trial will be conducted. In part 2, a two-armed (guided online CBT versus unguided online CBT) controlled trial will be conducted. Symptoms of PCBD, PTSD, and depression will be assessed via telephone interviews at pre-treatment/pre-waiting period, post-treatment/post-waiting period, and six months post-treatment. Potential participants are people who lost a loved one at least three months earlier during the COVID-19 pandemic with clinically relevant levels of PCBD, PTSD, and/or depression. Analysis of covariance and multilevel modelling will be performed. Discussion: This is one of the first studies examining the effectiveness of online grief-specific CBT. More research is needed before implementing online grief-specific CBT into clinical practice.