Seasonal adaptation to changes in light:dark regimes (i.e., photoperiod) allows organisms living at temperate latitudes to anticipate environmental changes. The circadian system has been implicated in measurement and response to the photoperiod. Yet, the key molecular pathways linking clock genes or the circadian clock to insect photoperiodic responses remain largely unknown. We showed that inactivating the clock in the North American monarch butterfly using loss-of-function mutants for circadian activators and repressor abolishes photoperiodic responses in reproductive output. Transcriptomic approaches in the brain of monarchs raised in long and short photoperiods, summer monarchs, and fall migrants revealed a molecular signature of seasonal-specific rhythmic gene expression that included several genes belonging to the vitamin A pathway. Rhythmic expression of these genes was abolished in clock-deficient mutants, suggesting that the vitamin A pathway operates downstream of the circadian clock. Importantly, a CRISPR/Cas9-mediated loss-of-function mutation in the gene encoding the pathway’s rate-limiting enzyme, ninaB1, abolished photoperiod responsiveness independently of visual function in the compound eye and without affecting circadian rhythms. Together, these results provide the first genetic evidence that the clock-controlled vitamin A pathway mediates photoperiod responsiveness in an insect, a function that could be evolutionary conserved in animals.