The aim of this study was to investigate the effect of male age on treatment outcomes and neonatal birthweight following intracytoplasmic sperm injection (ICSI). This study included 2,474 ICSI cycles. Male partners were stratified into 5-year age categories (up to 25, 26–30, 31–35, 36–40 and 41 and up). Multilevel logistic regression was used to evaluate the relationship between male age and treatment outcomes. After adjusting for confounders, we found no difference in the clinical pregnancy rate. However, we observed that the 31- to 35-year group had a higher odds of live birth than that of the >41-year group (aOR 1.63, p = .03), and that the risk of abortion in the 31- to 35-year group was lower than that of the reference group (aOR 0.41, p = .02). A total of 754 single-foetus newborns and 556 twin newborns were analysed. Among the singletons, none of the variables differed among the five groups (p > .05). Among the twins, the infants in the 36- to 40-year group had a lower neonatal birthweight and a higher low-birthweight rate than those of the other groups (p < .05). Our study indicates that increased paternal age negatively affects the live birth and miscarriage rates. In addition, advanced paternal age may affect the birthweight of twins.