The present study was designed to evaluate the effect of dietary protein and energy supplementation on the reproductive physiology, semen production, semen quality and fertility of goat bucks under the humid tropical island ecosystem of the Andaman and Nicobar Islands. A total of twenty-four healthy bucks was randomly divided into four dietary groups: control (ICAR standard ration), 20% more protein (MP), 20% more energy (ME) and a combined supplementation of 20% more protein and energy (MPE). The experiment was conducted under the semi-intensive management system across the rainy and dry summer seasons. The scrotal and testicular biometrics, endocrine profiles, antioxidant status, sexual behaviour, semen production, semen quality parameters and fertility rate were evaluated. The result revealed that the bucks treated with combined supplementation (20% MPE) had shown significantly (p < 0.05) higher body weight, scrotal circumference, testicular volume and testicular weight as compared with the other groups. Endocrine profiles revealed that higher concentrations of FSH, LH, testosterone, TSH, T3 and T4 along with lower cortisol and prolactin levels were observed in the MPE group. Antioxidant markers such as TAC, SOD, CAT and GSH were significantly enhanced whereas malondialdehyde (MDA) levels and oxidative stress index were significantly reduced. The sexual behaviour scores such as libido and mating ability were also significantly improved. Semen characteristics such as volume, sperm concentration, motility, viability, acrosomal integrity, plasma membrane integrity and nuclear integrity were higher in the MPE group whereas the sperm abnormalities and lipid peroxidation were lower. The rainy season was more favourable for the reproductive performance as compared to the stressful dry summer season. The in-vivo fertility trials revealed that higher kidding rates and greater numbers of kids were observed in animals inseminated with semen from MPE-treated bucks under both farm and field conditions. The results revealed that dietary supplementation with 20% additional protein and energy effectively mitigated the seasonal stress, improved the endocrine balance and antioxidant defence and enhanced the semen quality and fertility rate in goat bucks under humid tropical island conditions. This nutritional strategy was served as a practical approach to improve the reproductive efficiency and semen preservation in caprine species of tropical humid island ecosystems.