© 2022 Elsevier B.V.The objective of this study was to evaluate the effect of grape seed extract (GSE) supplementation on growth performance, blood metabolites, hormone responses, and fecal volatile fatty acids (VFA) concentration in dairy calves during the summer months. Sixty Holstein female calves 3 d old and 40.6 ± 2.17 kg body weight (BW) were randomly divided into four groups (n = 15). GSE was given orally to the calves with milk daily at doses of 0 (CON), 25 (GSE1), 50 (GSE2), and 100 (GSE3) mg/kg BW. Calves were weighed at 15-day intervals and blood samples were taken on d 3, 33, and 63. Fecal samples were collected on d 33 and 63 of the study to evaluate VFA. Temperature humidity index (THI) values ranged from 73.2 to 87.4 throughout the study, and the study was terminated on d 63. Calves fed GSE2 had the highest (P < 0.01) average daily gain (ADG) between d 3–18 and 33–48 of the study, while the GSE3 group had the highest (P < 0.01) ADG between d 18–33 and 48–63. Moreover, calves fed GSE3 had the highest (P < 0.01) starter mixture intake over the entire study period. Serum non-esterified fatty acids (NEFA) concentrations decreased (P = 0.003) in calves fed GSE2 compared to the other groups. Plasma insulin concentration decreased (P < 0.001) in calves fed GSE3 compared to the other groups, however, increased (P < 0.01) in GSE1 and GSE2 groups until d 33. Plasma cortisol concentration decreased (P < 0.01) in GSE2 and GSE3 groups compared to the CON and GSE1 groups. Also, calves fed GSE2 and GSE3 had lower (P < 0.01) leptin concentration than the others (day 63). Calves fed GSE3 increased (P < 0.01) fecal total VFA, acetate, propionate, and valerate concentrations compared to other groups, whereas the lowest (P < 0.05) concentrations were observed in the GSE1 group. In summary, GSE supplementation caused a beneficial effect on energy metabolism, endocrine system, and colonic fermentation and thus improved growth performance in Holstein calves under heat stress.