Contradictory reports regarding the effects of nanoselenium (NanoSe) on the performance of broiler chickens may occur. Therefore, the optimum supplementation of NanoSe doses needs to be determined. The current meta-analysis study was aimed at evaluating the effectiveness and the optimum doses of NanoSe supplementation in broiler diets on performance, blood constituents, carcass, and giblet weight by considering breed and sex. The database was obtained from online scientific publications by searching through search engines such as Scopus, Web of Science, Google Scholar, and PubMed by entering the keywords nanoselenium, performance, antioxidants, and broiler. A total of 25 articles were included in the meta-analysis database. The study group was treated as a random effect while NanoSe dose, breed, and sex were treated as fixed effects. Daily body weight gain, carcass weight, and breast weight increased quadratically (P < 0.05), and FCR decreased quadratically (P < 0.05) in the starter and cumulative periods with increasing NanoSe supplementation. NanoSe supplementation tended to decrease cumulative feed intake linearly (P < 0.1) and decreased (P < 0.05) abdominal fat, albumin, red blood cells, ALT, and MDA levels. In contrast, levels of total protein, globulin, glucose, AST, white blood cells, cholesterol, triglyceride, and the weight of the liver, heart, gizzard, bursa of Fabricius, thymus, and spleen were not affected by NanoSe supplementation. Increasing the dose of NanoSe increased (P < 0.05) the GSHPx enzyme and Se concentration in breast muscle and liver and tended to enhance (P < 0.01) the CAT enzyme. It is concluded that a proper dose of NanoSe supplementation in a broiler diet improves body weight gain, feed efficiency, carcass, and breast weight without adverse effects on giblets. Dietary NanoSe elevates Se concentration in the breast muscle and liver and antioxidant activity. The current meta-analysis shows that the optimum dose for body weight gain and FCR is 1 to 1.5 mg/kg.