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Simple Summary: Understanding the circadian rhythm of activity–rest in marine fish is essential for comprehending their ecological and evolutionary adaptations. By using a new tracking method to extract locomotory data from free-living marine fish, we have opened up new possibilities for studying the biology, behavior, and ecology of many other species in their natural conditions. Furthermore, this tracking method can help researchers identify how different environmental factors, such as temperature and light, affect the circadian rhythm of activity–rest in marine animals. This information can be used to predict the response of marine animals to changes in their environment and, ultimately, help guide conservation efforts. We found that the activity–rest circadian rhythm of Xyrichtys novacula was well synchronized by the environmental cycle of light–darkness regardless of gender and the period studied. However, the reproductive season caused direct effects on the rhythm, resulting in increased fragmentation and decreased synchronization in both genders. Recent technological advances in marine biotelemetry have demonstrated that marine fish species perform activity–rest rhythms that have relevant ecological and evolutionary consequences. The main objective of the present report is to study the circadian rhythm of activity–rest of the pearly razorfish, Xyrichtys novacula in its own habitat, before and during the reproduction season using a novel biotelemetry system. This fish species is a small-bodied marine species that inhabits most shallow soft habitats of temperate areas and has a high interest for commercial and recreational fisheries. The activity of free-living fish was monitored by means of high-resolution acoustic tracking of the motor activity of the fish in one-minute intervals. The obtained data allowed the definition of the circadian rhythm of activity–rest in terms of classical non-parametric values: interdaily stability (IS), intradaily variability (IV), relative amplitude (RA), average activity during the most-active period of consecutive 10 h (M10), and average activity during the least-active period of consecutive 5 h (L5). We observed a well-marked rhythm, with little fragmentation and good synchrony with the environmental cycle of light–darkness, regardless of sex and the period studied. However, the rhythm was found to be slightly more desynchronized and fragmented during reproduction because of variations in the photoperiod. In addition, we found that the activity of the males was much higher than that of the females (p < 0.001), probably due to the peculiar behavior of the males in defending the harems they lead. Finally, the time at which activity began in males was slightly earlier than it was in females (p < 0.001), presumably due to the same fact, as differences in activity or for the individual heterogeneity of this species in the time of awakening are considered to be an independent axis of the fish's personality. Our work is novel, as it is one of the first studies of activity–rest rhythm using classical circadian-related descriptors in free-living marine fish using locomotory data facilitated by novel technological approaches. [ABSTRACT FROM AUTHOR]