Stabilization and dewatering methods for wastewater solids determine the concentration and nature of phosphorus (P) in biosolids and in-plant sidestreams recycled to the liquid treatment facility. Because water treatment residuals (WTR) exhibit strong immobilization of soluble P, this study evaluated the impact of co-dewatering WTR and biosolids on the P partitioning during dewatering and the environmental lability of biosolids-P measured by water extractable P (WEP). Overall, P progressively partitioned into the water-insoluble particulate-bound form in dewatered cake with increasing blending ratio (BR)-defined as the dry mass ratio of WTR-to-biosolids. The reject water total P (TP) content from dewatering biosolids alone (250 mg L-1) was reduced to 60 mg L-1 for a BR=1.5. Polymer addition resulted in statistically (α=0.05) lower reject liquid TP, suggesting the cationic polyelectrolyte contributed to P binding. The WEP of the dewatered cake (~20% solids) dropped from 2.36 g kg-1 (biosolids only) to ~0.14 g kg-1 for BR = 1.5, meaning the P in land-applied co-processed cake is less susceptible to solubilization by surface runoff compared to unamended biosolids. Co-dewatering can reduce P in return flows and fix P in the dewatered solids in a form less prone to off-site migration following land application.