Milk yield, content and composition are altered by heat stress (HS), however, rumen-protected methionine feeding may ameliorate the effects of HS. Thirty-two multiparous, lactating Holstein cows [DIM (184 ± 59); body surface area (5.84 ± 0.34m²)] were randomly assigned to 1 of 2 environmental treatment groups, and 1 of 2 dietary treatments [TMR with rumen-protected methionine (RPM; Smartamine M; Adisseo Inc., Antony, France; 0.105% DM of TMR as top dress) or TMR without RPM (CON)] in a crossover design. The study was divided into 2 periods with 2 phases per period. In phase 1 (9d), all cows were in thermoneutral conditions (TN; THI = 60 ± 3) and fed ad libitum. In phase 2 (9d), group 1 (n = 16) was exposed to HS using electric heat blankets (THI = 89 ± 3). Group 2 (n = 16) remained in TN (THI = 61 ± 4) but was pair-fed (PFTN) to HS counterparts. After a 21d washout period, the study was repeated (period 2) and the environmental treatments were inverted relative to treatments from phase 2 of period 1, while dietary treatments remained the same. Cows were milked 3 × per d and samples were taken on d 1, 5, and 9 of each phase. Vaginal temperature was measured every 10 min, and respiration rate recorded once daily. Paired difference values were calculated for each cow for each period based on the difference between phase 1 baseline values and phase 2 values for each variable. Statistical analysis was performed on paired difference values using MIXED procedure of SAS. Cows in HS had greater (P < 0.001 and P < 0.001, respectively) increase in vaginal temperature and respiration rate (+0.2°C and +13.7 breaths/min respectively) compared with cows in PFTN (0.0°C and −1.6 breaths/min, respectively). Cows in PFTN had greater (P = 0.001 and P < 0.001, respectively) decrease in DMI and milk yield (−3.9 kg/d and −2.6 kg/d, respectively) compared with cows in HS (−3.2 kg/d and −0.9 kg/d, respectively). Cows in CON had greater (P = 0.04) decrease in milk protein proportion (%) for PFTN (−0.10%) and HS (−0.06%) compared with cows in RPM for PFTN (0.00%) and HS (−0.02%). In conclusion, HS altered physiological and production parameters, while RPM aided milk content and composition during HS.