This study aimed to evaluate the effect of humic acid (HA) on growth performance, nutrient digestibility, ruminal fermentation, ruminal enzyme activity, and certain blood biochemical parameters. A total of 45 calves (11 ± 0.25 months of age; 280 ± 5.55 kg BW) were divided equally into three groups. The control group received a basal diet composed of a concentrate feed mixture (CFM) with roughage. The treatment groups received the same basal diet supplemented with 1 and 2% humic acid in the CFM, respectively. The study revealed that the treatment groups had significantly lower total dry matter (DM) intake (p = 0.029). The treatment groups showed higher (p = 0.013) ether extract (EE) digestibility and digestible crude protein (DCP) (p = 0.001) than the CFM group. However, ammonia nitrogen (NH3-N, mg/100 mL) was significantly lower (p = 0.012) in the 1% HA and 2% HA groups compared to the CFM group. The 2% HA group had significantly higher (p = 0.001) proportions of acetate and propionate compared to the 1% HA and CFM groups. The treatment groups had significantly higher (p = 0.001) α-amylase, lipase, urease, and protease activities compared to the CFM group. In addition, the concentration of total soluble protein in the rumen liquid was higher (p = 0.006) in the 1% HA group compared to the CFM group. The 2% HA group had higher (p = 0.049) glucose concentrations than the CFM group. In conclusion, humic acid supplementation at 1% or 2% may improve nutrient digestibility and ruminal fermentation efficiency, enhance enzyme activity, and support normal blood profiles in calves.