The aim of this study was to evaluate the growth performance and plasma antioxidant activity of meat goats fed diets containing mangosteen peel powder (MPP; 41.25 g/kg dry matter). Twelve goats (body weight, 23.1 ± 1.49 kg) were fed a total mixed ration for 0–38 days under a severe temperature–humidity index (temperature, 80.96–103.64°F; humidity, 63%). Body weight and samples of feed, rumen fluid, blood, faeces, and urine were analysed. The MPP diet included condensed tannins, flavonoids, and cinnamic acid. Compared to the control group (CTL; no MPP), dietary MPP increased the nutrient digestibility of organic matter and body weight percentage of goats. Total volatile fatty acids, with a higher proportion of acetic acid, were higher in the MPP group; however, pH and NH₃-N remained unchanged for both groups. Methane production in the rumen was reduced in the MPP group. Dietary MPP partly improved biochemical indicators such as plasma total protein (PTP) and albumin (ALB), and modulated antioxidant activity by increasing the glutathione peroxidase concentration and 2,2-diphenyl-1-picrylhydrazyl scavenging activity in plasma. These findings indicate that feeding meat goats the proposed MPP diet represents an environmentally friendly strategy for improving growth performance and plasma antioxidant activity under a severe temperature–humidity index.