Some species of red seaweed have been shown to reduce methane emissions by over 80% in vitro and in sheep. The objective of this study was to determine the effective dose of Asparagopsis armata and quantify its effect on enteric methane yield (i.e., methane per unit of DMI), DMI, milk production, and feed conversion efficiency (FCE) in lactating dairy cattle in vivo. Twelve post-peak-lactation multiparous Holstein cows producing 34.7 ± 5.9 kg/d milk were randomly allocated to 1 of 3 treatments in a 3 × 3 Latin square design. The experiment contained 3 periods (14 d of measurement with 7 d of washout in between). Cows were fed the same basal diet with treatment 1 used as control and treatments 2 and 3 supplemented with tetrasporophyte phase of A. armata at 0.5% [low supplement (LS)], or 1% [high supplement (HS)] on a grams OM seaweed/grams OM basis. The doses were determined through a pretrial in which the cows were fed 0, 0.25, 0.5, 1, and 1.5% of A. armata. There was a significant (P < 0.001) reduction of enteric methane above 0.5%; however, above 1%, DMI was reduced. Effects of A. armata on enteric methane yield, DMI, and milk production were determined using the lmer function for linear mixed model analysis in R statistical software (version 3.4.4). Supplementation of A. armata resulted in a significant linear reduction in enteric methane yield (P < 0.001). The LS and HS treatments reduced enteric methane yield by 16 and 48%, respectively. Hydrogen emission (per unit of DMI) significantly (P < 0.001) increased by 57 and 306% compared with control for the LS and HS treatments, respectively. Carbon dioxide production as a proportion of DMI also increased 5.7 and 31.5% (P < 0.01) in LS and HS treatments, respectively. There was a linear decrease of 7.4 and 31.3% (P < 0.001) in DMI for LS and HS treatments, respectively. However, there was no reduction in milk production (P = 0.56). Consequently, FCE increased linearly (P < 0.001) with the HS diet increasing FCE 49.5%. This is the first study in dairy cattle supplementing the seaweed A. armata, which was shown to reduce enteric methane yield by >40% at 1% inclusion rate and significantly increase the FCE.