Background: In multiple sclerosis (MS), subtle disease progression starts early despite initial presentation. Early neurologic damage can be difficult to assess during infrequent clinical exams, making patient?centered care critical to optimal disease management. Floodlight MS was developed and tailored to meet the needs of people living with MS (PLwMS) using a Double Diamond (DD) model, which is based on convergent-divergent thinking and is commonly used in software development, to characterize the unmet needs of PLwMS and then design a suitable and feasible digital solution. PLwMS needs concerned gaining a broader understanding of their health status and better communication with healthcare professionals (HCPs). Objectives: To describe the use of rapid prototyping and human factors studies (HFS) in developing Floodlight MS. Methods: In the first phase of the DD, the overarching goal for Floodlight MS was defined as a solution that provides longitudinal patient data on key functional domains to facilitate better conversations between PLwMS and their HCPs. The second phase of the DD translated this goal into a digital solution using concepting (divergent thinking) to arrive at 4 design concepts, which were then converged through HFS to optimize and refine them. In rapid prototyping, the 4 concepts were presented to 9 PLwMS, then subsequently refined into a single concept in 2 further iterations with 9 PLwMS, and 11 PLwMS and 4 neurologists, respectively. HFS (12 PLwMS and 15 neurologists) then assessed Floodlight MS v1.0 usability and any risks associated with use, with attention to regulatory guidelines associated with software as a medical device (SaMD) developmentthe regulatory framework for digital health solutions. Results: Rapid prototyping and HFS were used to iterate a digital health concept for MS care, ensuring usability and acceptability, by optimizing elements such as user interface and tone of voice, data visualization, configuration and settings, as well as safe and effective use according to the SaMD regulatory framework; this required finding the right balance between desirability and feasibility. The outcome was Floodlight MS, a smartphone app that enables remote assessment of patient function and provides registered SaMD. Conclusions: Rapid prototyping and HFS were successfully used to develop Floodlight MS and led to an SaMD that is patient-centric in design but identified several trade-offs between patient needs, HCP expectations, technologic constraints and regulations.