The purpose of the present study was to examine the effects of an oral β-hydroxybutyrate (BHB) supplement on cycling performance. Using a double-blind, placebo-controlled, crossover design, 12 highly-trained cyclists (mean ± SD: age; 35 ± 8 y, mass; 74.5 ± 7.6 kg, VO₂ₚₑₐₖ; 68.0 ± 6.7 ml.min⁻¹kg⁻¹) were supplemented with two 30 ml servings of an oral BHB supplement or placebo formula (PLA) prior to and during exercise. Participants cycled at a submaximal intensity (80% of second ventilatory threshold) for 90-min, followed by a 4-min maximal cycling performance test (4PT). The difference in 4PT power output between trials was not statistically significant (p > 0.05) and was associated with a trivial effect (ES ±90%CI = 0.19 ±0.37). Ingestion of the BHB supplement was associated with a large increase in blood BHB concentrations when compared to PLA for the 4PT (ES = 1.75 ±0.50, p < 0.01). The increased BHB concentration was accompanied by a moderate increase in the respiratory exchange ratio (RER) during the submaximal exercise phase (ES = 0.54 ±0.45, p = >0.05) and a moderate increase during the 4PT (ES = 0.78 ±0.57, p = 0.03). Submaximal VO₂ did not differ between trials, however, VO₂ was higher during the 4PT phase in the BHB trial (ES = 0.28 ±0.32; small). In conclusion, BHB supplementation altered blood BHB concentrations, RER and VO₂ values during steady state sub-maximal exercise, but did not improve 4-minute cycling performance.