<p>Coastal gillnet fisheries allow fishers to target multiple species, thus increasing their ability to handle variability and uncertainty. However, gillnets can incur high bycatch of threatened marine megafauna. One emerging strategy to reduce marine megafauna bycatch is net illumination, which is thought to utilize differences in behavior or visual capacity between bycatch and target species. Although testing of net illumination has expanded into multiple coastal gillnet fisheries over the past decade, its effects have only been studied at night. To address this important knowledge gap, we compared the effects of net illumination on total and species-specific bycatch, target fish catch, and catch value across day and night periods in a coastal gillnet fishery along Mexico’s Baja California peninsula with among the highest reported marine megafauna bycatch rates worldwide. Using paired trials, we found that gillnet illumination significantly reduced total fisheries, elasmobranch, Humboldt squid, and finfish bycatch at night, with no reductions observed during the day. Furthermore, we found that the magnitude of nighttime bycatch reductions was significantly greater than the magnitude of daytime reductions for total fisheries and elasmobranch bycatch. By contrast, target fish catch and value were maintained across each illumination and time of day treatment. These results improve our understanding of this emerging bycatch reduction technology, while providing a framework to assess the effects of net illumination across day and night periods in passive net fisheries worldwide.</p>