This study was conducted to assess the impact of mannan-oligosaccharide (MOS) on growth performance, body physiology and tissue morphology of gilthead seabream (Sparus aurata). Treatment of fish with MOS-feed shown a significant increase in live weight and protein efficiency rates when were directly compared with mock-treated fish control. However, there was no statistically supported level of significance was observed for growth rates and feed conversion rates among groups. Improved live weight and protein efficiency rates reflected positively on the survival rate in MOS-fed fish. Interestingly, the whole body and fillet fatty acid composition shown no-correlation between treated and control groups (p>0.05). During the course of whole body examination, a positive correlation between MOS-fed and control-fed fish was observed for monounsaturated fatty acids and polyunsaturated fatty acids. However, these observations were not apparent in fillet samples. Profiling of the hepatic fatty acid clarified insignificant differences between MOS or mock treated groups for saturated fatty acids, monounsaturated fatty acids and polyunsaturated fatty acids values. Histological examination of fish that were fed on a diet supplemented with MOS shown no adverse effects on investigated organs, intestine and the liver. Taken together, it is plausible to state that a diet supplemented with MOS has positive effects on the survival rate and the fatty acids profile without any observable negative impact on body tissues and thus support the safe use of MOS in fish feed.