Objective: Hoarding disorder (HD) is difficult to treat. In an effort to increase efficacy and engagement in cognitive-behavioral therapy (CBT), we developed and evaluated a novel intervention comprising group CBT combined with between-session Internet-based clinician support for people with HD. Method: Twenty participants with HD received group CBT combined with an Internet-support system enabling therapist–participant communication between group sessions. Results: The treatment was associated with a significant reduction on the Saving Inventory—Revised (SI-R) and a large effect size (Cohen’s d = 1.57) was found at posttreatment. Treatment gains were maintained at the 3-month follow-up. Group attendance was high and no participants dropped out from treatment prematurely. Between-session motivational support from the therapist was most frequently mentioned as the main strength of the system. Conclusion: The results of this study support adding Internet-based clinician support to group CBT for HD to increase treatment adherence and, potentially, improve the overall efficacy of CBT.