Background Internet-based treatments for child anxiety may help to increase access to evidence-based therapies; however, user engagement, uptake, and adherence within routine clinical practice remain as challenges. Involving the intended end users in the development process through user-centered design and usability testing is crucial for maximizing user engagement and adoption of internet-based treatments, but so far this has been lacking for internet-based treatments for child anxiety. Objective The aim of this study is to develop an internet-based treatment for child anxiety through a process of user-centered design (phase 1) and usability testing (phase 2), based on an existing evidence-based, face-to-face, therapist-supported, parent-led cognitive behavioral therapy intervention. It is intended that the internet-based version of this treatment would consist of a parent website, case management system for clinicians, and mobile game app for children. Methods Parents, children, and clinicians who were familiar with the face-to-face version of the treatment were recruited from 2 National Health Service clinics. In phase 1, participants participated in 3 workshops to gain feedback on the overall concept, explore their wants and needs for the websites and game, generate ideas on how the treatment may look, and gain feedback on initial mock-ups of the websites and game. In phase 2, participants attended 3 individual usability testing sessions where they were presented with working prototypes of the website or game and asked to perform a series of tasks on the website (parents and clinicians) or play the game (children). The frequency and details on usability errors were recorded. Participants were asked for their feedback on the website and game using a standardized usability questionnaire and semistructured interviews. The websites and game were iterated after each round of usability testing in response to this feedback. Results In phase 1, participants approved the general concept and rated the initial mock-ups of the website and game positively. In phase 2, working prototypes were rated positively and usability errors declined across the iterations and were mainly cosmetic or minor issues relating to esthetic preference, with few issues regarding ability to navigate the website or technical issues affecting functionality. Feedback from the semistructured interviews further supported the positive response of participants to the website and game, and helped identify areas for improvement during the iteration process. The final iteration of the website and game are presented. Conclusions Taking an iterative approach to development through user-centered design and usability testing has resulted in an internet-based treatment for child anxiety (Online Support and Intervention for child anxiety) that appears to meet the needs and expectations of the intended users (parents, children, and clinicians) and is easy and enjoyable to use.