Health systems responsiveness is a key health systems goal, operationalised as an outcome measured across domains such as dignity and confidentiality. It also reflects values and inputs towards improved health. In this realist synthesis, we critically reviewed underpinning theories, examined mechanisms, and propose a theoretical model of health systems responsiveness. Four theories enhance the understanding of responsiveness: Complex Adaptive Systems, Human Agency, Health Equity, Justice and Social Accountability, and Cultural Capital. It is a social construct reflecting what people expect from the system within social and cultural contexts; and what systems actors (providers, managers) expect from people in the context of standards of care and available resources. Responsiveness is shaped by the societal context of care and the health systems context. Domains of responsiveness are inter-related and comprise values, processes and resources. Our proposed theory highlights the importance of favourable social and organisational contexts in triggering sense of agency, literacy and empowerment that contribute to enhanced people’s capacity to engage with health systems and health system’s capacity to respond to people’s expectations. We hope it offers a useful heuristic to inform efforts in improving health systems responsiveness.