An in vitro and an in vivo study were conducted to investigate the effects of a blend of cinnamaldehyde, eugenol, and capsicum oleoresin (CEC) on rumen fermentation parameters, animal performance, and methane (CH₄) emissions in dairy cows. Continuous culture fermenters (CCF) were utilized to test one of two treatments: (1) CON; no supplementation and (2) CEC supplemented at 0.0125 g/d. The basal diet consisted of grass hay and concentrate (50:50). Supplementation with CEC increased (p < 0.01) total volatile fatty acids (VFA; mM) and decreased (p = 0.02) CH₄ concentration compared with CON in vitro. Additionally, protozoa abundance tended (p = 0.07) to decrease in CEC compared with CON. The in vivo experiment utilized forty Holstein-Friesian dairy cows (32% primiparous and 68% multiparous) averaging 163 ± 48 days in milk (DIM) and 38 ± 6.2 kg/d of milk yield (MY). Cows were blocked by parity and randomly assigned to one of two treatments: (1) CON; no supplementation and (2) CEC supplemented at 1.2 g/cow/d. The basal diet consisted of grass hay and concentrate (40:60). Individual CH₄ emissions were recorded using the sniffer technique. Dry matter intake (DMI) and eating rate were increased (p < 0.01; 3.6% and 5.2%, respectively), while feed efficiency decreased (p < 0.05) in CEC compared with CON. Additionally, CEC decreased (p = 0.02) CH₄ yield by 16.4% and tended to reduce daily CH₄ production (p = 0.09) and CH₄ intensity (p = 0.08) by 13.4% and 14.0%, respectively. Supplementing CEC decreased CH₄ concentration in vitro and CH₄ yield in vivo without negatively impacting performance parameters.