Dietary betaine supplementation improves water retention in steers and may influence lean-tissue deposition, while also acting as an osmolyte to help regulate cellular osmotic balance. This study investigated the interactions between shade and dietary betaine on carcass characteristics, tissue enzyme activity and gene expression in 48 feedlot steers during summer. Steers were randomly allocated to a 4 × 2 factorial design with the factors being dietary betaine (0, 10, 20 or 40 g) and shade (with and without shade) for 120 days. Tissue samples were obtained at slaughter and analysed for gene expression of heat shock proteins 70 and 90 (HSP70/90) and expression of heat shock factor 1 (HSF1), and enzyme activity of fatty acid synthase (FAS) and glycerol-6-phosphate dehydrogenase (G6PDH). Carcasses were evaluated for quality. Carcass weight at slaughter was not altered by shade (P = 0.18) but tended to be increased by dietary betaine (306 v. 314 kg, P = 0.09). The P8 backfat was not altered by shade (P = 0.43) or dietary betaine (P = 0.32), although there was a within dietary betaine effect whereby P8 backfat tended to be greater in steers fed 10 g compared with 40 g betaine/day (17.4 v. 14.5 mm, P = 0.06). Muscle pH at 1 h (5.97 v. 6.03, P = 0.01) and 2 h (5.73 v. 5.80, P = 0.04) post-slaughter was higher in shaded steers, and muscle pH at 1 h post-slaughter was higher in steers fed 10 or 20 g than those fed 40 g betaine/day (6.03 v. 6.03 v. 5.95, P = 0.005). Gene expression was not altered by betaine, while adipose tissues expressed more of each gene than muscle (P < 0.001). The mRNA expression of HSF1 and HSP90 was influenced by a shade × betaine interaction, although the direction of this interaction was irregular (P = 0.03 and 0.03, respectively). Adipose tissue FAS and G6PDH enzyme activity was unaffected by shade and betaine. The results of this study indicate that betaine supplementation may be a successful carcass modifier in growing feedlot steers during summer. Provision of shade during summer may reduce the rate of pH decline in the first 2 h after slaughter and reduce the risk of high rigor temperature.