<p>The circadian clock is an endogenous time-keeping mechanism that enables organisms to synchronize their behavior and physiology with their daily environmental cycles. Although circadian systems have been well-studied in chordates and arthropods, little is known about them in terrestrial mollusks. In this study, we investigated the locomotor activity rhythm of the three-band garden slug, <i>Ambigolimax valentianus</i> (Férussac) (Gastropoda: Stylommatophora: Limacidae), which can inflict significant damage to crops, fruits, and garden plants, particularly in moist environments and small farming systems. This species showed a nocturnal locomotor activity rhythm under light–dark cycles and a circadian rhythm in constant darkness. The rhythm was temperature-compensated and could be entrained to non-24-h light–dark cycles. These results revealed three critical properties of circadian rhythm: self-sustained oscillation, temperature compensation, and entrainment. Slugs could entrain their locomotor activity rhythms to light–dark cycles, but unexpectedly, not to temperature cycles. Eyeless slugs showed clear rhythmicity and maintained their ability to entrain to light–dark cycles, indicating that extraocular clocks and photoreceptors play a crucial role in regulating locomotor activity rhythms. These findings provide new insights into the circadian biology of this species, highlighting the importance of light as a primary entraining cue and revealing the presence of extraocular photoreceptors.</p>