This paper reflects upon the concept of social innovation and its role in islands’ energy transition. As isolated energy systems, islands typically depend on energy imports from the mainland and mostly use fossil fuels for electricity, heating, and transport, which are significant sources of carbon emissions. At the same time, islands have an abundance of locally available renewable energy sources (RES) at their disposal, which makes them ideal test-beds for energy transition, or the technology-based switch of the energy system, from fossil-based to renewable energy. However, new RES technologies must be incorporated into society and, thus, to enable successful decarbonisation, technological innovations must be coupled with social innovations. Different authors stress that energy transitions are not strictly technical but socio-technical since they are also comprised of policies, politics and other artefacts, not just technological. Nevertheless, the role of social innovation in local energy transitions is still under-studied, and this paper aimed to contribute to this lack of literature, focusing on the local energy transitions of islands. By combining theoretical and empirical research, this paper aims to explore the role of social innovation in energy transition and analyse whether social innovation can be considered a success factor in the energy transition process of the case-study island, the small Croatian island of Unije.