Internal phosphorus (P) released from sediments, which has been verified to be affected by iron (Fe) and sulfur (S) reactions, strongly influences P cycling and water eutrophication. However, minimal research has been performed to determine how increased low-level salinity in estuarine tidal marshes affects P mobility and its coupled processes. Herein, in situ high-resolution distributions of labile P, Fe, and S in sediment-overlying water profiles were simultaneously measured along an estuarine freshwater-oligohaline gradient (Min River Estuary, China) utilizing newly developed diffusive gradients in thin films technique. Significant variations in the P, Fe, and S concentrations were observed with significantly higher sediment labile P concentrations in the freshwater-oligohaline transition. The observed labile P and Fe coupling confirmed the Fe redox-driven P release mechanism in the sediment, although this coupling was weakened in the freshwater-oligohaline transition. The diffusion flux results revealed that the sediments shifted from P “sink” to “source” with the transition from a freshwater to an oligohaline environment, probably caused by the release of Fe-bonded P stimulated by sulfate reduction. This study determined that increased salinity and the associated environmental responses alter the P remobilization capacity and internal P cycling by changing the sediment P pool and the P-Fe-S couplings across the sediment–water interface. Furthermore, saltwater intrusion into tidal freshwater wetlands caused by strong typhoons or sea-level rise may increase internal P release from sediments, which could have future implications.