Although the use of ultraviolet (UV) filters in personal care products is steadily increasing, their ecological consequences remain poorly characterized despite evidence of persistence and bioaccumulation in marine systems. In parallel, climate change stressors such as rising temperatures and fluctuations in salinity are known to modulate the toxicity of contaminants and the physiological tolerance of marine organisms. The combined action of these factors can intensify biological stress, highlighting the need for studies that evaluate pollutant effects under realistic multi-stressor scenarios. This study investigated the biochemical effects of octinoxate (also known as ethylhexyl methoxycinnamate, EHMC), a widely used organic UV filter, on the sea urchin Paracentrotus lividus under environmentally relevant conditions. A 28-day laboratory exposure was conducted using three EHMC concentrations (50, 500, and 5000 ng/L) under control conditions (17 °C, salinity 35) and climate change scenarios (21 °C, salinity 40). Multiple biomarkers were analysed, including metabolic activity, antioxidant and biotransformation responses, redox balance, cellular damage, and neurotoxicity. Results showed that the biochemical responses of P. lividus were significantly influenced by environmental conditions. Combined exposure to EHMC and elevated temperature (21 °C) induced marked oxidative stress, metabolic alterations, and shifts in detoxification responses. These effects were less pronounced under increased salinity, though still detectable. The present findings emphasize the heightened vulnerability of marine invertebrates to chemical pollutants under climate stress. Furthermore, the present study highlights the importance of integrating multiple stressors into ecotoxicological assessments and supports the use of bioindicator species, such as P. lividus, for more realistic environmental risk evaluations. Given that UV filters remain understudied contaminants and that data on their effects in echinoderms are still almost nonexistent, this work provides a timely contribution and highlights a critical knowledge gap that warrants urgent scientific attention.