Titanium dioxide nanoparticles (TiO2-NPs) are used intensively. Thanks to their extremely small size (1–100 nm), TiO2-NPs are more absorbable by living organisms; consequently, they can cross the circulatory system and then be distributed in various organs including the reproductive organs. We have evaluated the possible toxic effect of TiO2-NPs on embryonic development and the male reproductive system usingDanio rerioas an organism model. TiO2-NPs (P25, Degussa) were tested at concentrations of 1 mg/L, 2 mg/L, and 4 mg/L. TiO2-NPs did not interfere with the embryonic development ofDanio rerio, however, in the male gonads the TiO2-NPs caused an alteration of the morphological/structural organization. The immunofluorescence investigation showed positivity for biomarkers of oxidative stress and sex hormone binding globulin (SHBG), both confirmed by the results of qRT-PCR. In addition, an increased expression of the gene responsible for the conversion of testosterone to dihydrotestosterone was found. Since Leydig cells are mainly involved in this activity, an increase in gene activity can be explained by the ability of TiO2-NPs to act as endocrine disruptors, and, therefore, with androgenic activity.