Twenty Dorper × Katahdin male lambs (body weight = 33.9 ± 0.4 kg and age = 4.5 months) were individually housed for a 30-d feeding period to evaluate the effects of seasonal heat stress (winter [n = 10] vs. summer [n = 10]) on feedlot performance, carcass traits, wholesale cut yields and meat quality. Heat stress environmental conditions prevailed in summer and mostly thermoneutral in winter. Overall growth rate and feed efficiency, as well as empty body weight at slaughter, cold carcass weight, omental fat percentage, and loin yield were lower in summer than in winter. Kidney-pelvic-heart fat deposition and yields of hot carcass, neck and shoulder increased during summer. Postmortem aging of meat during 14 d exhibited higher lightness, redness, yellowness and toughness in summer. In conclusion, summer heat stress decreased growth and feed efficiency without affecting feed intake of hair male lambs. Additionally, heat stress improved carcass yield with no detrimental changes on meat quality.Copyright © 2020 Elsevier Ltd. All rights reserved.