Heat stress impairs spermatogenesis primarily through increased testicular oxidative stress. Astaxanthin, a potent antioxidant, has shown beneficial effects in sperm cryopreservation. However, its potential to mitigate testicular damage induced by elevated temperatures remains uninvestigated. In this study, male mice were administered astaxanthin (10 mg/kg/d, 50 mg/kg/d, and 100 mg/kg/d) via gavage for 21 days, with heat exposure occurring during the final 14 days. Samples were collected after the last treatment or following a recovery period. Spermatozoa in the cauda epididymis were assessed using computer-aided sperm analysis (CASA) or Diff-Quik staining, and serum testosterone levels and oxidative stress markers in both serum and testis were quantified via enzyme-linked immunosorbent assay (ELISA). Heat treatment resulted in significant reductions in sperm motility, viability, and morphological integrity. However, daily supplementation of astaxanthin at 50 mg/kg and 100 mg/kg effectively alleviated these heat-induced impairments. Furthermore, astaxanthin at 50 mg/kg/d notably improved testis weight, testis index, and serum testosterone levels under heat stress. Its antioxidant capacity was validated by significant restoration of total antioxidant capacity (T-AOC), superoxide dismutase (SOD), and reduction in malondialdehyde (MDA) levels in both testis and serum. In conclusion, this study highlights the protective effects of astaxanthin against heat-induced testicular and sperm damage by reducing oxidative stress, supporting its potential use as a nutritional or nutraceutical supplement to promote male reproductive health, particularly in the context of summer subfertility in farm animals.