The purpose of this study was to characterize mangrove-derived organic matters in wetland sediments and to establish a quantitative differentiation between mangrove-derived organic matter and other organic matter source(s) in the Zhangjiang Estuary mangrove wetland, southern China. Total organic carbon, total nitrogen, carbon/nitrogen ratio, and stable carbon isotopic compositions were measured in surface sediments, three sediment cores (in pure forest of Kandelia candel, Aegiceras corniculatum, and Avicennia marina, respectively), and mangrove plant samples collected from mangrove wetlands. A mixing model was applied to calculate the contribution of the mangrove-derived organic matter in sediment. The mangrove-derived organic matter in three sediment cores from the mangrove wetland were distinguished quantitatively. Our results indicated that organic matter preserved in the sediments was not predominantly composed of the mangrove-derived organic matter in the study areas. The in situ contribution of mangrove plant carbon to sediment organic matter differed between the three core samples. The composition of the mangrove-derived organic matter is 6.36%, on average, in K. candel pure forest, which is much lower than the values in A. marina and A. corniculatum pure forest (21.95% and 36.88%, respectively).