Increasing physical activity (PA) through exercise is associated with improvements in many of the symptoms associated with multiple sclerosis (MS) such as fatigue, strength, balance, and mobility. Despite this, people with MS remain largely inactive. Interventions that are grounded in theory and that aim to change PA behavior need to be developed. The purpose of this study was to describe the development process of a web-based resource, namely, "Activity Matters," to enable people with MS to become more active. Development of the "Activity Matters" online intervention was guided by the UK's Medical Research Council (MRC) framework for the development and evaluation of complex interventions and the behavior change wheel (BCW). Seven sources of data were used to inform the process and were mapped on to both the MRC and BCW frameworks. The intervention is theoretically based, and constructs including knowledge, memory, attention and decision processes, skills, social influences, environmental context and resources, beliefs about capabilities, beliefs about consequences, goals, and emotions were recognized as important. "Activity Matters" is the first MS PA intervention to be developed using the theoretical approach outlined by the BCW and MRC complex interventions frameworks. The next phase of this work is to test the usability, acceptability, and preliminary effectiveness of "Activity Matters" among people with MS.