Abstract The diverse fauna and flora of rocky intertidal ecosystems are being impacted by the activities of rapidly increasing coastal populations in many regions of the world. Human harvesting of intertidal species can lead to significant changes in body sizes of these taxa. However, little is known about the temporal trajectories of such size declines and more importantly, the long-term effects of chronic human impacts. Furthermore, it is unclear whether sizes of species not directly targeted for harvesting are also declining through indirect effects. Here we use historical (extending back to 1869) and field survey data covering 200 km of mainland southern California coast to show that human activities have led to significant and widespread declines in body sizes of rocky intertidal gastropod species over the last century. These declines, initiated several decades ago, are continuing and contrary to expectation, they are not restricted to species harvested for human consumption. Data from the only national park in this area, where conservation laws are strictly imposed, demonstrate that negative ecological impacts can be ameliorated if existing laws are enforced.