Bioretention cells are a Low Impact Development (LID) technology that is being promoted as a green solution to attenuate urban stormwater nutrient loadings. Despite extensive implementation of bioretention cells in Canada, the mechanistic understanding of phosphorus (P) cycling in bioretention cells is still limited. We conducted detailed analyses of (geo)chemical and hydrological data coupled to numerical reactive transport modeling to simulate the fate and transport of P in a bioretention cell located in Mississauga (Ontario, Canada) within the Credit River watershed. Our objective is to utilize the model to predictively understand the accumulation and speciation of P in the bioretention cell under long-term field operation. Unlike existing bioretention models, our model incorporates a detailed representation of the biogeochemical processes that control P cycling in the bioretention cell. We further compare the model predictions with data from sequential chemical extractions of P from soil samples taken from the bioretention cell. The model correctly estimates the cumulative TP (total P) and SRP (soluble reactive P) outflow loadings from the bioretention cell, as well as the TP accumulation rate and observed partitioning of P over the different pools in the bioretention cell. The relative importance of various processes controlling P retention are assessed using mass balance calculations and sensitivity analyses of the model. The results show that filtration of fine P-containing particles and slow sorption are the main processes retaining P in the bioretention cell.