Reverse osmosis (RO) desalination has become a prominent desalination method for fresh-water production from either saline seawater or brackish water to meet the ever-growing demand for water, especially in water-scarce regions. Its integration with renewable energy sources (RES) reduces the environmental impact of carbon emission by conventional fossil fuel energy sources. The optimisation of the RES-RO desalination system is intended mainly to minimise total system cost and energy requirements and to guarantee system reliability. In this study, an extensive review of the optimisation of the RES-RO desalination system is presented based on optimal system sizing, optimal system operation and optimal thermodynamic analysis. RES such as geothermal, ocean, wind and solar energy and their hybrids were considered alongside desalination methods. Important findings of the review were discussed, and recommendations made for future work. Key recommendations of this study include the suggestion that extensive optimisation and analysis of a RES-RO system should utilise optimisation approaches that combine the sizing, operation and thermodynamic effect of the system. Future work should furthermore incorporate both economic and reliability indices in the formation of objective functions. Finally, demand response programmes can be introduced to the RES-RO system for demand side management. This has the potential of minimising system cost while maximising fresh-water production.