Soil P availability has been identified as one of the key factors controlling wetland productivity, structure, and function. Soil P fractions at different depths in newly formed wetlands along a salinity gradient in Yellow River Delta (China) were studied using a modified Hedley fraction method. The total P (Pt) content ranged from 471.1 to 694.9 mg kg–1, and diluted HCl-extractable inorganic P (Dil-HCl-Pi) ranged from 324 to 524.2 mg kg–1. The Dil-HCl-Pi is the predominant P form in all profiles, with on average 70% of the Pt extracted as Pi. Organic P (Po) comprised (4.2 ± 2.0)% (mean ± SD) of the Pt, due to low organic-matter content in coastal salt marsh ecosystems. The labile P (resin-P, NaHCO3-Pi, and NaHCO3-Po) and moderately labile P (NaOH-Pi and NaOH-Po) concentrations were both low, ranged from 11.6 to 38.1 and 2.8 to 21.3 mg kg–1, respectively, constituting (3.7 ± 1.1)% and (2.0 ± 0.7)%, respectively, of Pt, suggesting low availability of P to plants in these soils. Our results suggested that vegetation cover significantly influenced soil P dynamics and availability. In particular, the labile P content under Tamarix chinensis increased significantly by 23.2%–145.5% compared with adjacent soils. These findings have important implications for wetland conservation or restoration and long-term sustainable management of newly formed wetland ecosystems in the Yellow River Delta.