Dietary tannin supplementation represents a potential strategy to modulate rumen fermentation and enhance lactation performance in dairy cows, though responses remain inconsistent. A 21-day feeding trial was conducted to evaluate the effect of chestnut tannin (CNT) extract on the enteric methane emissions (EME), blood metabolites, and milk production traits in mid-lactation dairy cows. Thirty-six Holstein cows were allocated to three homogeneous treatment groups: control (CNT, 0 g/d CNT), CNT (40 g/d CNT), and CNT (80 g/d CNT). Measurements of EME, dry matter intake (DMI), milk yield (MY), and blood and milk parameters were carried out pre- and post-21-day supplementation period. Compared with the no-additive group, the CNT extract reduced methane production, methane yield, and methane intensity in CNT and CNT ( < 0.001). CNT and CNT cows exhibited lower blood urea nitrogen ( = 0.019 and = 0.002) and elevated serum insulin ( = 0.003 and < 0.001) and growth hormone concentrations ( = 0.046 and = 0.034), coinciding with reduced aspartate aminotransferase ( = 0.016 and = 0.045), and lactate dehydrogenase ( 0.011 and 0.008) activities compared to control. However, CNT had higher circulating NEFA and BHBA than CNT ( = 0.003 and = 0.004) and CNT ( = 0.035 and = 0.019). The blood glucose, albumin, and total bilirubin concentrations were not affected. MY and fat- and protein-corrected milk (FPCM), MY/DMI, and FPCM/DMI were higher in both CNT ( = 0.004, = 0.003, = 0.014, = 0.010) and CNT ( = 0.002, = 0.003, = 0.008, = 0.013) cows compared with controls. Feeding CNT resulted in higher protein content ( = 0.015) but lower fat percentage in milk ( = 0.004) compared to CNT. Milk urea nitrogen and somatic cell counts were significantly lower in both CNT ( < 0.001, = 0.009) and CNT ( < 0.001 for both) compared to CNT, while milk lactose did not differ between treatments. These findings demonstrate that chestnut tannin extract effectively mitigates EME while enhancing lactation performance in mid-lactation dairy cows.