<p>Artificial light at night (ALAN) creates unnaturally bright nighttime environments. While ALAN provides various societal benefits, it also exerts negative effects on animal activities, particularly those regulated by the circadian light–dark cycle, such as daily activity timing. Although previous studies have shown that ALAN can extend temporal activity in diurnal vertebrates, little is known about its effects on daily activity timing in diurnal invertebrates. Here, we address this gap by examining the effects of ALAN on nighttime acoustic signaling in the diurnal katydid <i>Conocephalus chinensis</i>, using both field surveys and laboratory manipulative experiments. We first recorded the presence or absence of signaling males during the day and night at lit plots exposed to streetlamps and at dark plots across four areas in central Hokkaido, northern Japan. We then sampled ten males from each area and recorded their signaling at night under experimentally controlled light–dark cycles with LED lamps. Field surveys revealed that katydids were active both during the day and at night in plots exposed to streetlamps. Laboratory experiments also confirmed that light treatment increased nighttime signaling. Together, these findings provide the first evidence that ALAN induces signaling activity in a diurnal insect at night, with effects that would be similar to those documented in vertebrates. Such alterations in signaling behavior could have important ecological consequences, potentially enhancing mating opportunities but also increasing predation risk and shortening lifespans, underscoring the need to better understand the impacts of light pollution on diurnal invertebrates.</p>