<p> The climbing perch, <i>Anabas testudineus</i>, is a widespread amphibious freshwater fish commonly found in shallow, often human-modified habitats across South and Southeast Asia. Several seasonal cycles—such as spawning and terrestrial movements—have been relatively well characterised, yet diel activity rhythms remain unstudied. This study aimed to characterise the diel pattern of locomotor activity of the climbing perch under natural light–dark cycles and assess changes in activity under additional constant artificial light, as typically encountered near populated areas. Under natural light (0–250 Lx; <i>n</i> = 64), fish exhibited an activity pattern with two peaks: 1 h after sunrise (06:00–07:00) and 1 h before sunset (17:00–18:00). Locomotor activity was highest under twilight conditions, suggesting predominantly crepuscular behaviour. Under constant artificial light (≤ 50 Lx, <i>n</i> = 64), this pattern was flattened: crepuscular peaks decreased, while fish activity increased during the second half of the night. Additionally, the timing of high- and low-activity periods differed between light regimes. These findings demonstrate that artificial illumination significantly alters the diel locomotor patterns in <i>A. testudineus</i>. The results contribute to understanding how light pollution affects diel rhythms in tropical freshwater fishes and highlight its ecological relevance in increasingly modified aquatic environments.</p>