BackgroundStroke survivors face many barriers to physical activity (PA). Solving physical inactivity after stroke requires a “systems-based” approach. We aimed to develop a complex intervention targeted at improving PA after stroke in Singapore using behaviour change theory and a co-design approach involving multiple stakeholders.MethodsWe carried out the intervention development in three phases: i. preparation phase, ii. co-design phase, and iii. intervention refinement phase. During the preparation phase, we conducted surveys (n = 38 stroke survivors, 71 physiotherapists and 35 exercise professionals) and interviews (n = 19 stroke survivors) to understand the factors influencing PA after stroke. The co-design phase consisted of two-co-design workshops held in August 2022 and were attended by 13 stroke survivors and 4 caregivers. Relevant domains of the Theoretical Domains Framework (TDF) and items in the Template for Intervention Description and Replication (TIDieR) checklist guided the discussion topics in the first co-design workshop. Solution prototypes such as exercise videos, arm and leg straps and information resources were shown in the second co-design workshop to gather feedback. In the intervention refinement phase, eight healthcare professionals from various sectors participated in two virtual Zoom meetings in August 2023, and used the Acceptability, Practicability, Effectiveness, Affordability, Spillover effects and Equity (APEASE) grid to rate the active ingredients in the complex intervention by considering the current healthcare landscape in terms of resources and manpower.ResultsStroke survivors and caregivers want a personalised PA program, stroke-specific PA opportunities and information resources, medical clearance, advice and help from healthcare professionals skilled in stroke care, face-to-face sessions at preferred exercise spaces, and access to adaptive equipment. A complex intervention consisting of 21 behaviour change techniques, 6 intervention functions and 8 options was developed.ConclusionsUsing behaviour change theory and a co-design approach involving multiple stakeholders, a complex intervention was developed to target physical inactivity after stroke. The intervention titled MOTIVATE is currently being tested in a type 1 hybrid effectiveness-implementation trial.