Maintaining optimal physical and cognitive performance are keys to success for most exercise contexts. However, consensus on the effects of dehydration for cognitive function is equivocal, particularly given the addition of confounding variables when hypohydration (HYPO) results. Therefore, this study aimed to examine whether maintaining euhydration (EUH) would preserve cognitive function, and whether this physiological state would be superior than if HYPO were evoked in an identical exercise task. In a crossover design, 15 participants (12 males, age 27.93 ± 6.81 years, height 177.20 ± 6.95 cm, mass 84.40 ± 12.35 kg) completed a 90 min self-paced simulated military march in the heat, whilst either maintaining EUH by consuming fluid ad libitum or becoming hypohydrated via fluid restriction. A cognitive testing battery was administered pre-exercise and following a rest period (55 ± 8 min), and evaluated information processing, memory, impulsivity, attention and concentration, and response time domains, whilst subjective estimates of performance were also quantified. Aspects of memory and impulsivity were not comparable to pre-exercise data (both P ≤ 0.05), whilst a shift in the speed-accuracy trade-off was apparent in the switching attention task, with accuracy decreasing (P = 0.003), and reaction time being supplemented (P = 0.028). Despite body mass losses of 2.28%, hydration status did not influence performance for any of the measured cognitive domains (all P > 0.05). When hypohydrated, subjective estimates of thirst were significantly greater post-exercise (P = 0.004), whilst medium effect sizes were found for lethargy (d = 0.532) and task difficulty (d = 0.553) post-exercise. Although maintaining EUH by en-large preserves cognitive function, this does not produce superior cognitive performance compared with fluid restriction following an identical exercise task. Therefore, despite losses in body mass exceeding 2%, cognitive performance remains largely stable.