Copyright and Photocopying: © 2015 The Authors. Conservation Letters published by Wiley Periodicals, Inc. Many marine ecosystems are shaped by regimes of natural light guiding the behavior of their constituent species. As evidenced from terrestrial systems, the global introduction of nighttime lighting is likely influencing these behaviors, restructuring marine ecosystems, and compromising the services they provide. Yet the extent to which marine habitats are exposed to artificial light at night is unknown. We quantified nighttime artificial light across the world's network of marine protected areas (MPAs). Artificial light is widespread and increasing in a large percentage of MPAs. While increases are more common among MPAs associated with human activity, artificial light is encroaching into a large proportion of even those marine habitats protected with the strongest legislative designations. Given the current lack of statutory tools, we propose that allocating “Marine Dark Sky Park” status to MPAs will help incentivize responsible authorities to hold back the advance of artificial light.