Background Decentralisation is a common mechanism for health system reform; yet, evidence of how it impacts health systems remains fragmented. Despite published findings from primary and secondary research illustrating range of impacts, a comprehensive and clear understanding is currently lacking. This review synthesised the existing evidence to assess how decentralisation (by devolution) impacts each of the six WHO building blocks, and the health system. Method We systematically searched five electronic databases for reviews exploring impact of decentralisation on health systems, globally. Reviews, both systematic and non-systematic, published in the English language from January 1990 to February 2022 were included. Data were synthesised across each of six building blocks. Quality assessment of the reviews was conducted using Critical Appraisal Skills Program for systematic and Scale for Assessment of Narrative Review Articles for non-systematic reviews. Results Nine reviews, each addressing somewhat different questions, contexts and issues, were included. A range of positive and negative impacts of decentralisation on health system building blocks were identified; yet, overall, the impacts were more negative. Although inconclusive, evidence suggested that the impacts on leadership and governance and financing components in particular shape the impact on overall health system. Assessment of how the impact on building blocks translates to the broader impact on health systems is challenged by the dynamic complexities related to contexts, process and the health system itself. Conclusions Decentralisation, even if well intentioned, can have unintended consequences. Despite the difficulty of reaching universally applicable conclusions about the pros and cons of decentralisation, this review highlights some of the common potential issues to consider in advance. PROSPERO registration number CRD42022302013.