Background: The rapid increase in the number of elderly family caregivers underlines the need for new support systems. Internet-delivered psychological interventions are a potential approach, as they are easy to access for family caregivers who are often homebound with their care recipient. This study examines the relative effectiveness of an internet-based acceptance and commitment therapy (ACT) intervention or a standardized institutional rehabilitation program, first, in reducing depressive symptoms, and second, in improving the well-being and quality of life of elderly family caregivers compared to a control group receiving support from voluntary family caregiver associations. Methods: 156 family caregivers aged 60 or more are studied in a quasi-experimental study design that compares three groups of family caregivers (Group 1; n = 65: a guided 12-week web-based intervention; Group 2, n = 52: a standardized institutional rehabilitation program in a rehabilitation center; Group 3, n = 39: support provided by voluntary caregiver associations). Data collection is performed at three time-points: pre-measurement and at 4 months and 10 months thereafter. Caregivers’ depressive symptoms as a primary outcome, and perceived burden, anxiety, quality of life, sense of coherence, psychological flexibility, thought suppression, and personality as secondary outcomes are measured using validated self-report questionnaires. Physical performance and user experiences are also investigated. Between-group differences in the effects of the interventions are examined using multiple-group modeling techniques, and effect-size calculations. Discussion: The study will compare the effectiveness of a novel web-based program in reducing depressive symptoms and improving the psychological well-being of elderly family caregivers, or a standardized institutional rehabilitation program representing usual care and a control group receiving support offered by voluntary caregiver associations. The results will expand the knowledge base of clinicians and provide evidence on effective strategies to improve the mental health and overall quality of life of elderly family caregivers. Trial registration: The study was retrospectively registered in www.clinicaltrials.gov (ClinicalTrials.gov Identifier: NCT03391596 on January 4, 2018.