METHODS/DESIGN: This will be a randomized wait-list controlled internet-delivered trial. A cognitive behavioral-therapy program for the concerned significant others (CSOs) of problem gamblers (PGs) will be developed and evaluated. The participants will work through 8 modules over 10 weeks in a secure online environment, and receive support via text messages and over telephone. 150 CSOs over 18 years of age will be included. Measures will be taken at baseline 3, 6 and 12 months. Primary outcomes concern gambling-related harm and motivating the PG to seek treatment. Secondary outcomes are CSO's feelings of depression, anxiety, relationship satisfaction and quality of life.