Methionine is the first limiting amino acid for milk and protein production in lactating dairy cows fed corn-based diets. A field trial was conducted in a commercial dairy farm in Northeast Spain with the aim to evaluate the effect of supplementing a rumen-protected methionine product (Timet; Vetagro S.p.A.; Reggio Emilia, Italy) on lactation performance in high-yielding dairy cows. Ninety-nine multiparous Holstein cows were used in a 3-period switchback design: Control 1 (CTR1, 21 d), Timet (TMT, 21 d) and CTR2 (12 d). All the cows that entered the study were between 0 and 120 DIM. Cows were fed a TMR formulated to provide 16.7% CP, 29% starch, and 32% NDF with 2793 g/d of MP and Lys:Met 2.93. During TMT treatment, Timet (55% of DL-Methionine) was supplemented 25 g/d per head. Milk yield was recorded daily and milk was sampled for protein, fat, and urea every 2 d. Data were analyzed using MIXED model ANOVA including cow as random effect (JMR pro 13). Although there was not significant difference in milk yield, milk protein was significantly increased by TMT (3.43%) compared with CTR1 (3.35%) and CTR2 (3.38%) (P < 0.05), and milk fat was also significantly increased by TMT (3.75%) compared with CTR1 (3.63%) and CTR2 (3.71%) (P < 0.05). During TMT treatment it was possible to observe a trend in lowering milk urea (180.8 mg/L) compared with CTR1 (193.0 mg/L) and CRT2 (189.9 mg/L) (P = 0.43). In conclusion, addition of Timet to a corn-based diet increased milk protein and fat concentration.