Simple SummaryNatural infections caused byAeromonas veroniiin intensive farming can lead to economic losses in tilapia farming. Overusing antibiotics and chemical antimicrobial agents in fish farming leads to antibiotic resistance, pollution, and consumer reluctance. The utilization of mangosteen (Garcinia mangostana) peel extract loaded in nanoemulsion (MSNE)-supplemented diets in Nile tilapia (Oreochromis niloticus) could improve growth performance, immune response, and disease resistance. Nevertheless, the effect of incorporating MSNE into Nile tilapia diets has not yet been studied. In this study, we assessed the efficacy of MSNE-supplemented diets on growth performance, immune response, and resistance toA. veroniiinfection in Nile tilapia. The particle size, polydispersity index, and particle surface charge of MSNE were 151.9 ± 1.4 nm, >0.3, and −30 mV, respectively. Furthermore, MSNE improved the in vitro inhibition againstA. veronii,and MSNE-supplemented diets had a beneficial effect on growth performance, enhanced immune response, and disease resistance againstA. veroniiinfection. In conclusion, mangosteen peel extract loaded in nanoemulsion has the potential to be used as a supplement in Nile tilapia culture.AbstractNanotechnology can enhance nutrient delivery and bioavailability; hence, it has recently been considered the most practical alternative technology for nutritional supplements and disease control in fish farming. The present study was designed to evaluate the effects of mangosteen peel extract loaded in nanoemulsion (MSNE) on the inhibition ofA. veronii(in vitro) and in vivo growth performance, serum biochemical parameters, the immune response, and the disease resistance of Nile tilapia (Oreochromis niloticus) againstA. veroniichallenge. The particle size, polydispersity index, and particle surface charge of MSNE were 151.9 ± 1.4 nm, >0.3, and −30 mV, respectively. Furthermore, MSNE, mangosteen peel extract (MPE), and nanoemulsion (NE) improved the antimicrobial activity againstA. veronii. Fish fed MSNE, MPE, and NE-supplemented diets had a significantly lower (p< 0.05) feed conversion ratio (FCR) and higher specific growth rate (SGR) than fish fed the control diet. Furthermore, the MSNE had significantly higher serum glucose and protein levels than the control group in Nile tilapia. Total immunoglobulin, serum lysozyme, alternative complement activity, and survival of Nile tilapia fed with MSNE were significantly higher (p< 0.05) than the control diet. Therefore, MSNE has the potential to be employed as a supplement in sustainable Nile tilapia farming.