<p>Urban Heat Island (UHI) intensifies urban heat stress and cooling demand, and roofs are among the most solar-exposed surfaces in dense cities. Roof-integrated phase change materials (PCMs) can store daytime heat as latent energy and release it during cooler periods, thereby reshaping diurnal heat transfer rather than solely reducing it. This systematic review synthesises peer-reviewed studies (2010–2025) on PCM-integrated roof systems, covering PCM types and encapsulation, roof configurations, and experimental and numerical evidence across climates relevant to UHI mitigation. Across the literature, the dominant benefit is temporal heat regulation, including attenuation of peak roof-surface temperature, delayed heat transmission, and partial peak-load shifting, while annual energy savings are often secondary and context-specific. Performance is strongly conditional on (i) aligning PCM phase-transition temperature with local roof temperature cycles, (ii) sufficient nocturnal regeneration, and (iii) coordination with roof optical and thermal properties; benefits diminish under persistently warm nights and constrained urban ventilation. Key gaps include long-term outdoor durability under cycling, harmonised evaluation metrics, and robust attribution of neighbourhood-scale microclimatic impacts. Overall, PCM roofs should be positioned as climate-responsive complements to cool roofs and integrated blue-green strategies, with design priorities centred on temperature matching, regeneration feasibility, and system-level optimisation.</p>