Background: Diazinon is an insecticide with oxidative stress properties, which is widely used in agriculture. Selenium is used as a basic nutrient for animals and humans as a potential antioxidant. No studies have evaluated the protective effects of selenium on diazinon in the reproductive system of male Wistar rats. Objectives: The present study aimed to assess the therapeutic effects of selenium on balanced testosterone levels, sperm DNA damage, fertilization, and early embryonic development in diazinon-treated rats. Methods: This experimental study was conducted on 32 mature male Wistar rats, which were randomly assigned to four groups of eight, including control (0. 2 mg/kg/B. W of corn oil), diazinon (300 mg/kg/B. W), selenium (0. 5 mg of Na2SeO3/kg), selenium (0. 5 mg/kg), and diazinon (300 mg/kg/B. W of diazinon). The animals received daily oral treatment for 60 days. Results: Compared to the control group, the diazinon group showed a significant reduction in the testosterone levels, sperm count, sperm motility, sperm maturation, DNA maturity, in-vitro fertilization (IVF), two-cell embryo, hatching, and blastocyst (P value < 0. 05). However, the sperm mortality, sperm DNA damage, and immature sperm significantly increased in the treatment group (P value < 0. 05). In the diazinon and selenium treatment group, the total sperm parameters returned to the physiological range, and no significant changes were observed. Conclusions: According to the results, selenium could protect the sperm parameters, testosterone level, and IVF in the diazinontreated Wistar rats.