The discharge of phosphorus-rich saline wastewater deteriorates the water quality and causes frequent public health incidents. Therefore, this kind of wastewater must be treated using clean and efficient methods. The main objective of this study is to investigate the phosphorus removal performance in constructed wetlands (CWs) with intertidal wetland sediments (IWS) as a new microbial source for treating saline wastewater. IWS was introduced into the substrate of CWs planted with Phragmites australis. CWs without IWS and plants were set as the control groups. The phosphorus removal via plant uptake, substrate accumulation, and microbial transformation were evaluated, and the microbial community was analyzed under salty environment. Results showed that the removal rates in CWs with IWS (89.03% ± 1.47%) were higher than those in CWs without IWS (82.35% ± 1.58%). The microbial analysis revealed that numerous bacteria actively related to phosphorus removal, such as phosphorus-accumulating organisms, denitrifying PAOs, and phosphate solubilizing bacteria, were present in IWS-CWs. These bacteria enhanced the phosphorus removal through microbial transformation. Moreover, the addition of IWS increased the phosphorus loss via substrate accumulation, whereas the phosphorus removal rate via plant uptake decreased. The microbial augmentation of CWs with IWS as a new inoculation source is eco-friendly, economical, and effective and provides the possibility of applying CWs in the field of saline wastewater treatment.