Dairy cows experiencing heat stress (HS) during the pre-calving portion of the transition period give birth to smaller calves and produce less milk and milk protein. Supplementation of rumen-protected methionine (RPM) has been shown to modulate protein, energy, and placenta metabolism, making it a potential candidate to ameliorate HS effects. We investigated the effects of supplementing RPM to transition cows under HS induced by electric heat blanket (EHB) on cow-calf performance. Six weeks before expected calving, 53 Holstein cows were housed in a tie-stall barn and fed a control diet (CON, 2.2% Met of MP) or a CON diet supplemented with Smartamine®M (MET, 2.6% Met of MP, Adisseo Inc., France). Four weeks pre-calving, all MET and half CON cows were fitted with an EHB. The other half of the CON cows were considered thermoneutral (TN), resulting in 3 treatments: CONTN (n = 19), CONHS (n = 17), and METHS (n = 17). Respiratory rate (RR), skin temperature (ST), and rectal temperature (RT) were measured thrice weekly and core body temperatures recorded bi-weekly. Post-calving body weights (BW) and BCS were recorded weekly, and DMI was calculated and averaged weekly. Milk yield was recorded daily and milk components were analyzed every third DIM. Biweekly AA and weekly nonesterified fatty acids (NEFA), β-hydroxybutyrate (BHB), insulin, and glucose were measured from plasma. Calf birth weight and 24 h growth, thermoregulation, and hematology profile were measured and apparent efficiency of absorption (AEA) of immunoglobulins was calculated. Data were analyzed using the MIXED procedure of SAS with 2 preplanned orthogonal contrasts: CONTN vs. the average of CONHS and METHS (C1) and CONHS vs. METHS (C2). Relative to TN, EHB cows had increased RT during the post-calving weeks and increased RR and ST during the entire transition period. Body weight, BCS, DMI, and milk yield were not impacted by the EHB or RPM. However, protein % and SNF were lower in CONHS, relative to METHS cows. At calving, METHS dams had higher glucose concentrations, relative to CONHS, and during the post-calving weeks, the EHB cows had lower NEFA concentrations than TN cows. Calf birthweight and AEA were reduced by HS, while RR was increased by HS. Calf withers height tended to be shorter and RT were lower in CONHS, compared with MTHS heifers. Overall, RPM supplementation to transition cows reverts the negative impact of HS on blood glucose concentration at calving and milk protein % in the dams and increases wither height while decreasing RT in the calf., (© 2024, The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)