Manganese, an essential nutrient for male reproductive health, exerts dose-dependent effects, with excessive exposure-particularly to manganese dioxide nanoparticles (MnO(2)-NPs) from environmental or industrial sources inducing gonadal damage via oxidative stress, hormonal disruption, and impaired steroidogenesis. This study evaluated rosemary essential oil (REO) against MnO(2)-NP-induced reproductive dysfunction in male rats. Seventy-two Sprague-Dawley rats (130 ± 10 g) were divided into six groups (n = 12): Group I (deionized water), Group II (saline), Group III (MnO(2)-NP, 100 mg/kg bw/day), Group IV (REO, 250 mg/kg/day), Protective Group V (REO pre-treatment + MnO(2)-NPs), and Therapeutic Group VI (MnO(2)-NPs + REO co-treatment) for 56 days. MnO(2)-NP exposure caused testicular injury, marked by elevated lipid peroxidation (↑malondialdehyde, ↑nitric oxide), suppressed antioxidants (↓total antioxidant capacity, ↓catalase, ↓glutathione), impaired sperm parameters (motility, count, morphology), and altered serum hormone levels (follicle-stimulating hormone, luteinizing hormone, testosterone). These effects correlated with downregulated steroidogenesis genes (StAR, HSD-3β, CYP11A1). Both Protective and Therapeutic REO treatment mitigated MnO(2)-NPs oxidative stress, restored hormonal balance, and normalized gene expression. Histopathology revealed reduced seminiferous tubule degeneration and enhanced spermatogenesis in REO groups. Findings demonstrate REO's efficacy in alleviating MnO(2)-NPsinduced reproductive toxicity via antioxidant and steroidogenic modulation, positioning REO as a promising therapeutic against nanomaterial-induced gonadotoxicity.