The endangered Galápagos sea lion lives among a rapidly growing human population, and conflicts between humans and sea lions are increasing. Protection of this fragile species requires a better understanding of how anthropogenic activity affects its health and survival. In this study, we engaged a group of local students in a community science project to conduct long-term observations of the effects of human disturbance on sea lion behavior. We compared three types of behavior – reaction to human approach, vocalizations and group size – across four different haul-out sites which varied in their levels of human disturbance. We found that sea lions respond less aggressively to humans on beaches that are more disturbed. This may be because sea lions acclimate to human disturbance or because sea lions with a low tolerance for humans avoid disturbed sites. We also found that aggressive vocalizations between sea lions increase as sea lion group size increases, though group size was not linked to human disturbance. We did not quantify stress levels, but aggressive behavior often indicates elevated stress levels, which are energetically costly and can impair immune function. Our results suggest that conservation efforts should focus on limiting human–sea lion interactions and increasing the number and quality of available haul-out sites.