Abstract  [Objectives]  To determine the circadian eclosion rhythm of Spodoptera frugiperda in response to photoperiod cues and establish an eclosion behavior-based assay to explore the endogenous circadian clock of this pest. [Methods]  S. frugiperda were kept under a L14︰D10 (LD) photoperiod before being randomly subdivided into one of four treatment groups; constant darkness (dark-dark, DD), constant light (light-light, LL), and two groups with an extra day of exposure to either constant darkness (DD+1) or constant light (LL+1). The control group was kept under the original LD photoperiod for an entire generation. Nine-day-old (for DD+1 and LL+1 groups) or 10-day-old (for LD, DD, and LL) pupae were transferred to a purpose built, chronobiology-based, full-time monitoring device for determining the biological rhythm phenotype of small animals. Pupal duration, eclosion duration, and circadian eclosion rhythm were monitored and determined by combining different photoperiod cues. [Results]  Adult eclosion mainly occurred in the dark phase, or the corresponding phase of the dark phase, during LD entrainment under the LD, DD, DD+1, and LL photoperiods. The adult emergence peak occurred at Zeitgeber Time ZT15-16 (24%) under LD. Moreover, anticipation of the light-to-dark transition for eclosion was found during the light phase under LD. Adult emergence peaks in the DD and DD+1 groups were delayed, occurring at Circadian Time CT17-18 (24%) and CT21-22 (22%), respectively. Although poor eclosion rhythm was still evident under LL, an extra day of exposure to LL (i.e., LL+1) completely disrupted the circadian eclosion rhythm of S. frugiperda. [Conclusion]  A circadian eclosion rhythm exists in S. frugiperda that is regulated by both light-dark cycle entrainment and an endogenous circadian clock. The adult eclosion peak occurs during the dark phase of a long day photoperiod, and the free-running circadian period underlying the circadian eclosion rhythm of S. frugiperda is longer than 24 hours. Overall, adult circadian eclosion rhythms appear to be a reliable phenotype that can help reveal circadian clockwork mechanisms in S. frugiperda.