Abstract Background Although Internet-based interventions (IBIs) have been around for two decades, uptake has been slow. Increasing the acceptability of IBIs among end users may increase uptake. In this study, we explored the factors that shape acceptability of IBIs for problem gambling from the perspective of clients and clinicians. Findings from this qualitative study of focus groups informed the design and implementation of an IBI for problem gambling. Methods Using a semi-structured interview guide, we conducted three focus groups with clients experiencing gambling problems (total n = 13) and two with clinicians providing problem gambling treatment (total n = 21). Focus groups were audio recorded, transcribed verbatim, and analyzed using a two-part inductive-deductive approach to thematic analysis. Results Although both user groups reported similar experiences, each group also had unique concerns. Clinician perspectives were more homogeneous reflective of healthcare professionals sharing the same practice and values. Clinicians were more concerned about issues relating to the dissemination of IBIs into clinical settings, including the development of policies and protocols and the implications of IBIs on the therapeutic relationship. In comparison, client narratives were more heterogeneous descriptive of diverse experiences and individual preferences, such as the availability of services on a 24-h basis. There was consensus among clients and clinicians on common factors influencing acceptability: access, usability, high quality technology, privacy and security, and the value of professional guidance. Conclusions Acceptability is an important factor in the overall effectiveness of IBIs. Gaining an understanding of how end users perceive IBIs and why they choose to use IBIs can be instrumental in the successful and meaningful design, implementation, and evaluation of IBIs.