Objectives: OpenCAT is an open-source computerized adaptive testing (CAT) web delivery platform built with reusability, accessibility, and adaptability in mind. CAT survey question sequences are dynamic and tailored to individual participants, presenting as few questions as possible while collecting all the necessary information. The aim of building this platform is to help patients assess their recovery progress, to allow clinicians and researchers to administer adaptive questionnaires and collect results for further analysis, and to let IT personnel easily deploy the system in a variety of environments— including when HIPAA requirements apply. Design: It is designed with accessibility in mind, and offers full Section 508 and WCAG 2.0 Level AA compliance to enable use within federally-funded research programs or when contracted by a federal agency. Furthermore, the user interface is optimized for use with both a touchscreen and a mouse, and adapts to any screen size (from smartphones to desktop computers). Architecture: For developers, it is written with modularity in mind. The platform allows for an easy replacement of item banks, incorporation of different adaptive testing algorithms, and delivery of different data visualizations, all without touching the core of the application. By making the platform open source and providing a RESTful API, while simultaneously enabling proprietary algorithms and item banks to remain private, we encourage widespread community adoption and reusability across different audiences and patient populations. OpenCAT can also be deployed as a Docker image, which makes the application portable, scalable, and platform-agnostic. Conclusions: OpenCAT has been utilized within the scope of two projects so far. We continue to incorporate feedback from researchers, organizations, and users, expanding the feature set. Planned future work includes a stand-alone version for deployment in settings without internet access.