Cardiorespiratory fitness is thought to be positively related to sustained attention. However, the underlying mechanisms of this relationship have yet to be fully elucidated. The objective of this study was to i) explore the relationship between cardiorespiratory fitness and sustained attention in 72 young adults (18–37 years old) and ii) provide insight on the electrocortical dynamics supporting sustained attention performance in individuals differing in cardiorespiratory fitness by means of EEG topographic analyses and source localization. Behaviorally, cardiorespiratory fitness was related to faster response times and higher accuracy in the psychomotor vigilance task even when adjusting the model with confounding variables such as age, body mass index and chronic physical activity. However, there was no relationship between cardiorespiratory fitness and the classic vigilance decrement observed in the sustained attention task. At the electrocortical level, higher cardiorespiratory fitness was related to increased global field power (310–333 ms poststimulus) localized in the posterior cingulate cortex (BA 30) followed by changes in scalp topographies around the P3b ERP component (413–501 ms poststimulus), which corresponded to earlier activation of the supplementary motor areas (BA 6). This is the first study using high-density EEG, which harnesses the whole spatiotemporal dynamics of the relationship between cardiorespiratory fitness and sustained attention in young adults.