Biochar, a carbon-rich product from the pyrolysis of biomass, is a potentially effective feed additive to improve diet digestion and gut health; however, its effect on the horse digestive system is unknown. Our goal was to evaluate the effects of supplementation of horse diet with biochar on total tract apparent digestibility of nutrients, fecal characteristics (pH, and concentrations of volatile fatty acids (VFA) and ammonia-N), and blood serum parameters (metabolites, minerals, and vitamins). Eight Warmblood horses (mean +/- SD; 613.8 +/- 39.3 kg of body weight, BW) were used in a crossover design with two 15-day periods. Each 15-day period had a 10-day adaptation phase and a 5-day collection phase. Treatments were: (1) Control, basal diet consisted of meadow hay and crushed barley (80:20 on dry matter (DM) basis), and (2) Biochar, basal diet supplemented with 10 g of biochar/kg of diet DM. The horses were fed a total of 15 g DM/kg BW. Data were analyzed as a crossover design using the MIXED procedure of SAS. There were no statistical differences in nutrient digestibility between treatments; however, digestibility of all nutrients, except starch, was numerically higher in horses fed a diet with biochar. Fecal pH was higher (P = 0.015) in horses fed the diet supplemented with biochar than in those fed the control diet (mean pH 6.49 and 6.34, respectively). Increased fecal pH might be due to the high alkalinity of the biochar used. Neither the fecal concentration of total VFA (P = 0.295) nor the proportion of individual VFA (P > 0.05) were affected by the treatment. Concentrations of fecal ammonia-N were also unaffected (P = 0.887) by biochar. Biochar supplementation had no effect (P > 0.05) on serum concentrations of metabolites, minerals, or vitamins. In conclusion, our results suggest that dietary inclusion of biochar can be a safe and low-cost measure to ameliorate acidic pH in the hindgut of horses. However, differences in individual susceptibility to biochar effect on fecal pH warrant further investigation.