The need to generate sustainable energy from renewable sources has driven the emergence of processes that use the sea as a supply source. Osmotic energy, in particular, is gaining strength as a promising alternative. This review aims to analyze the influence of different pretreatment technologies on the performance of Pressure Retarded Osmosis (PRO) processes in terms of power density generation. Using the PRISMA methodology, a systematic review of the research available in the Scopus and Web of Science databases between 2006 and 2022 was carried out. The results revealed that pretreatment processes are essential for the success of the PRO application and that different pretreatment technologies can lead to different efficiency levels in removing suspended solids and organic and inorganic compounds. It is also essential to calculate the specific energy costs required by the pretreatment system, as this aspect can significantly limit the feasibility of the application. This review contributes to identifying and addressing pretreatment as a critical area requiring further research and attention in optimizing this technology. It highlights the importance of customized pretreatment approaches and identifies future trends in advanced membrane development and energy efficiency. These trends will not only improve the economic viability of Pressure Retarded Osmosis but will also be critical in the transition to more sustainable and efficient energy production.