Manganese inhibits oxidative stress damage. The aim of this study was to investigate the protective role of manganese on testis structure and sperm parameters in adult mice exposed to formaldehyde (FA). Twenty adult male NMRI mice were selected and randomly divided into four groups: (i) control; (ii) sham; (iii) 'FA'-exposed group; and (iv) 'FA and manganese chloride'-exposed group. The FA-exposed groups received 10 mg kg(-1) FA daily for 14 days, and manganese chloride was just injected intraperitoneally 5 mg kg(-1) on 2nd weeks. Mice were sacrificed, and spermatozoa were collected from the cauda of the right epididymis and analysed for count, motility, morphology and viability. The other testicular tissues were weighed and prepared for histological examination upon removal. Seminiferous tubules, lumen diameters and epithelium thickness were also measured. The findings revealed that FA significantly reduced the testicular weight, sperm count, motility, viability and normal morphology compared with control group (P ≤ 0.05). In addition, seminiferous tubules atrophied and seminiferous epithelial cells disintegrated in the FA group in comparison with the control group (P ≤ 0.05). However, manganese improved the testicular structure and sperm parameters in FA-treated mice testes (P ≤ 0.05). According to the results, manganese may improve and protect mice epididymal sperm parameters and testis structure treated with FA respectively.