Doenjang (fermented soybean paste) is the traditional fermented soybean food which is widely consumed in Korea. Carbohydrate is one of the most abundant components in fermented soybean paste, and it plays important roles in nutrition, organoleptic quality and functionality. In order to understand its utilization by microorganisms in fermented foods, it is necessary to develop highly accurate analytical platform. However, currently available method cannot distinguish structrally similar monosaccharides. In this study, we developed and validated a sensitive and accurate UHPLC-MS/MS method for the simultaneous analysis of 15 monosaccharides in Doenjang. The linearity and sensitivity of this method were excellent; all monosaccharides had the high responses for linear regression coefficients (R² > 0.9995). The range of LOD was from 4.22 fmol for rhamnose to 126.75 fmol for fructose. The method accuracy ranged from 76 - 116%. We used the developed method to analyze free monosaccharides in a homogenized Doenjang sample. 12 monosaccharides were found. Galactose, glucose, arabinose and mannose were predominant, and xylose, fucose, rhamnose, galacturonic acid and N-acetylglucosamine were also found. The developed method can be readily adaptable to rapid analysis of monosaccharides in various fermented soybean foods.