People with multiple sclerosis experience barriers to physical activity. Thought processes are interwoven with garnering motivation to overcome these barriers. This study investigated in-depth the role of positive thinking in physical activity motivation of two women and two men with multiple sclerosis. Participants thought aloud while completing standardised measures of physical activity, stages of change and self-efficacy, and in response to planned and spontaneous questions. Four themes were formulated using inductive thematic analysis: thoughts about purpose, self-efficacy, the past and reinforcement through positive thinking. These findings have implications for physical activity theories and delivering appropriate physical activity interventions to the multiple sclerosis community.