The Juan Fern & aacute;ndez Archipelago (JFA) is a unique ecosystem due to its diversity and high endemism. In recent years, there has been an increase in the populations of the endemic sea urchin Centrostephanus sylviae, generating negative impacts on the ecological and socio-economic systems of the Archipelago. Here, we describe the establishment of C. sylviae and the consequences of its overabundance on the subtidal communities, specifically around Robinson Crusoe and Santa Clara islands. Also, we describe the variation in SST (degrees C) around both islands during the last 40 years. The increase in abundance of C. sylviae and its community-level impacts (i.e. sessile benthic, macroinvertebrates, and fishes) were documented by visual surveys using scuba diving at various sites during January 2014 (12 sites), March 2017 (12 sites), March 2020 (4 sites), and January 2024 (7 sites). Over this period, the increase of C. sylviae abundance (similar to 400%) was associated with a significant decrease in sessile biotic benthic cover and an increase in the percentage of barren grounds (40%-84%). Additionally, there was a significant decrease in the abundance of the sea star Patiriella calcarata (100%) and the sea cucumber Mertensiothuria platei (70%). In terms of fish, a decrease in the abundance of the endemic wrasse Malapterus reticulatus (95%), and an increase of the chilean sweeper Scorpis chilensis (40%) were observed. Both species evidenced a decrease in biomass (90% and 84%, respectively). In general, the abundance of C. sylviae could be attributed to the synergistic effects of both top-down and bottom-up processes, including the low presence of predators (Astrotole platei and Jasus frontalis), by the increase in nutrient concentrations, and by the influence of climate change. These results could serve as a baseline for exploring ongoing community changes in subtidal environments.