Abstract: The organic phosphorus (OP) in sediments of estuaries has a potential release risk, and the contents and distributions of OP forms are of crucial significance for evaluating eutrophication. However, so far, few studies have investigated the distribution characteristics of OP forms in sediments from estuary areas. In this study, the OP forms in sediments from the main estuaries of the Maowei Sea, northern South China Sea were analyzed by the sequential extraction procedures. Results showed that the average content of OP in surface sediments of Qin River Estuary (QRE) was (6.87±0.51) μmol/g, which was significantly higher than that of Maoling River Estuary (MRE) (average, (3.01±0.16) μmol/g). Nonlabile organic phosphorus (NLOP) predominated the sedimentary OP in the two main estuary areas, and the proportions of the three OP components in OP were in the order of NLOP＞moderately labile organic phosphorus (MLOP)＞labile organic phosphorus (LOP). Residual organic phosphorus (Res-OP) was the main OP form in sediments of the MRE, accounting for 67.86% of OP, followed by humic acid organic phosphorus (HA-OP) and fulvic acid organic phosphorus (FA-OP). HA-OP and Res-OP were the main OP forms in sediments of the QRE, followed by FA-OP. There is spatial heterogeneity in the vertical distribution of different OP forms in surface sediments of the main estuaries of the Maomei Sea. Biomass organic phosphorus (Bi-OP) was significantly positively correlated with total organic carbon (TOC) in the sediments, indicating that organic matter was an important carrier of Bi-OP. Hydrochloric acid organic phosphorus (HCl-OP) was significantly positively correlated with clay and silt, respectively, suggesting grain size was an important factor affecting the distribution of HCl-OP in the sediments. Sedimentary HA-OP was significantly positively correlated with dissolved inorganic phosphorus (DIP) and dissolved iron (DFe) in the pore water, respectively, indicating that disallogenic iron reduction can promote mineralization of organic matter and transform LOP into HA-OP. Sedimentary Res-OP was significantly negatively correlated with DIP, DFe and dissolved sulfur (DS) in the pore water, respectively, suggesting that Res-OP has potential biological activity and can release phosphorus into the water body, which is a significant part of migration and transformation from the sediments into the pore water, and its contribution to the eutrophication of estuaries and bays deserves attention. The results of this study provide some new insights for phosphorus biogeochemistry in sediments from estuaries of subtropical bays, China.