Heat stress-induced sperm DNA damage has recently been demonstrated in boars during tropical summer; which could negatively impact early embryo survival and litter size in sows. Given the boar's inefficient capacity to sweat, non-pendulous scrotum and low antioxidant activity in seminal plasma, elevated endogenous levels of antioxidants are needed to combat reactive oxygen species induced during periods of heat stress. This should prevent the build-up of pathological levels of DNA damage in boar spermatozoa. Our aim was to investigate whether a combined antioxidant supplement could mitigate sperm DNA damage in boars exposed to tropical summer conditions. Terminal deoxynucleotidyl transferase dUTP nick end labelling and flow cytometry of 20,000 spermatozoa/boar/treatment revealed that boar diets supplemented with 100 g/day custom-mixed antioxidant during peak wet summer effectively reduced sperm DNA damage by as much as 55% after 42 and 84 days treatment respectively (16.1 ± 4.9 peak wet control vs. 9.9 ± 4.5 42 day vs. 7.2 ± 1.6% 84 day treatments; P ≤ 0.05). Supplementation did not improve sperm concentration beyond control levels for either season (P > 0.05); nor alter total motility, progressive motility or several other motion parameters measured by computer assisted sperm analysis of 20 x 106 sperm/mL at 38°C (P > 0.05). Antioxidant supplementation during tropical summer appears to mitigate the negative impact of heat stress on DNA integrity but not concentration nor motility of boar spermatozoa; which may provide one solution to the problem of summer infertility in the pig.