Methionine (MET) is the first limiting amino acid (AA) in corn silage-based diets when metabolizable protein is supplied with soybean meal. The aim of this study was to determine the effect of rumen-protected MET (RPM) supplementation on productive performance and plasma AA concentration of lactating cows. Seventy-six multiparous Holstein lactating cows, housed in a freestall barn and milked 3× daily were divided in 2 groups and stratified by milk yield (MY) and DIM (39.1 ± 6.8 kg/d of milk and 65 ± 28 DIM). Cows were assigned to 1 of 2 treatments in a randomized complete block design with a 2-wk covariate period followed by a 10-wk treatment. Treatment diets were CON (n = 38) basal diet (3.77LYS:1MET); and MET balanced with an RPMet source, RPM (n = 38; basal diet + 23 g of Smartamine M, Adisseo SA; 2.97LYS:1MET). MY was recorded daily and individual milk samples were taken every 2 weeks throughout the experiment to determine milk composition and milk components yield. A subset of 24 cows had blood samples collected on +30 d to measure plasma AA concentration. Body weight (BW) and body condition score (BCS) were measured at the beginning and the end of the experiment. Data were analyzed using the MIXED procedure of SAS with repeated measures. MY (41.7 vs. 40.1 kg/d; P = 0.026), ECM (41.0 vs. 38 kg/d; P ≤ 0.01), milk protein yield (1.30 vs. 1.18 kg/d; P ≤ 0.01) and milk fat yield (1.42 vs. 1.29 kg/d;P = 0.02) were greater for RPM compared with CON group. Milk percentage of protein (3.14 vs. 2.97%; P ≤ 0.01) and casein (2.39 vs. 2.28%; P ≤ 0.01) were also greater for RPM with tendency (P = 0.06) for higher milk fat percentage compared with control (3.41 vs. 3.21%). Supplementation with RPM did not affect MUN and SCC. Both treatments had similar BW, but the CON tended (P = 0.08) to have higher BCS. Plasma Met concentration was greater for RPM compared with CON (29.6 vs. 18.4 μM; P < 0.01), with no differences observed in Lys, and His concentration. Supplementation with RPM improved productive performance by increasing milk yield and components, and this data suggests that dietary AA utilization with adequate level of LYS is improved by adjusting the MET level.