Despite recent advances in acute stroke intervention, secondary prevention strategies are lacking. Physical activity (PA) is the second largest predictor of stroke and the cornerstone of secondary prevention therapies. Adaptive interventions involve sequential treatments responsive to individuals' performance. Despite guidelines to adapt PA to individuals' needs, there is no evidence on the empirical development of adaptive PA interventions post-stroke. The evidence is dominated by standard trial designs, used to evaluate non-adaptive PA interventions. This trial will make original contributions to the literature by designing a first-in-class adaptive PA intervention using an innovative experimental design. This design will permit the delivery of optimal sequences of treatments to increase PA for individuals. Furthermore, by developing a user-informed smartphone application, this feasibility trial will lead to the design and evaluation of a highly-scalable definitive intervention to reduce the risk of recurrent stroke. Aim To use a Sequential Multiple-Assignment Randomised Trial (SMART) design to develop an adaptive, user-informed mobile health (mHealth) intervention to improve PA post-stroke. Subsequently this adaptive intervention will be evaluated against a treatment-as-usual control using a standard trial design in a definitive trial. Methods A co-design process was used to develop the technology platform for intervention delivery. There are two components which target increased PA in the current intervention: Structured Exercise and Lifestyle PA. The SMART will determine what sequence and/or combination of these intervention components produces the greatest increase in physical activity as measured by average daily step count. The primary outcome will be PA (steps/day), with clear feasibility and secondary clinical and cost outcomes. A SMART design will be used to evaluate the optimum adaptive PA intervention among people post-mild-to-moderate stroke.