New Findings: What is the central question of this study? Does a ketogenic diet (KD) modulate circulating counts of natural killer (NK) cells, including CD56bright and CD56dim subsets, and their ability to activate (CD69 expression) following in vitro antigen stimulation in response to exhaustive moderate-intensity exercise? What is the main finding and its importance? The KD amplified the biphasic exercise-induced NK cell response due to a greater mobilisation of the cytotoxic CD56dim subset but did not alter NK cell CD69 expression. The KD appears to modulate exercise-induced circulating NK cell mobilisation and egress, but not antigen-stimulated circulating NK cell activation. Abstract: We investigated the effect of a 31-day ketogenic diet (KD) compared with a habitual, carbohydrate (CHO)-based diet on total circulating natural killer (NK) CD3−CD56+, dim and bright subset count, and antigen-stimulated CD3−CD56+ cell activation (CD69+) in response to exhaustive running. In a randomised, repeated-measures, cross-over study, eight trained, male endurance athletes ingested a 31-day low-CHO KD or their habitual diet (HD). On day 31, participants ran to exhaustion at 70% (Formula presented.) (∼3.5–4 h, ∼45–50 km). A low-CHO (<10 g) meal was ingested prior to the KD trial, with fat ingested during exercise. A high-CHO (2 g kg−1) meal was ingested prior to the HD trial, with CHO (∼55 g h−1) ingested during exercise. Venous blood samples were collected at pre-exercise, post-exercise and 1 h post-exercise. The KD amplified the classical exercise-induced biphasic CD3−CD56+ cell response by increasing the post-exercise counts (P = 0.0004), which appeared to be underpinned by the cytotoxic CD3−CD56dim subset (main effect of time point, P < 0.0001). The KD had no effect on NK cells’ expression of CD69 or their geometric mean fluorescence intensity of CD69 expression, either for unstimulated or for antigen-stimulated NK cells (all P > 0.05). In conclusion, adaptation to a KD may alter the number of circulating NK cells but not their ability to activate to an antigenic challenge.