Abstract: Dairy processing sludge (DPS) represents a valuable source for phosphorus (P) recovery given its high P content (up to 50 mg-P/g-dw). However, the limited understanding of the P speciation hinders its valorisation. This study investigated the P speciation of DPS throughout a dairy processing wastewater treatment plant (WWTP), which employed chemical P removal (iron dosing) and enhanced biological P removal (EBPR). According to 31P nuclear magnetic resonance spectroscopy, the primary sludge was dominated with organophosphate (organic-P) which accounted for 74% of total P (TP), and the remaining P was inorganic orthophosphate (ortho-P, 25% of TP). After EBPR and iron dosing, the activated sludge was dominated with ortho-P (68% of TP), and the other P was biogenic-P, i.e., polyphosphate (poly-P, 22% of TP) and organic-P (10% of TP). With sedimentation, the content of poly-P decreased (14% of TP) probably due to its partial hydrolysis under anoxic condition in the clarifier, resulting in more ortho-P (77% of TP) in the final DPS. Sequential P extraction showed that the ortho-P was mainly Fe-P (63–70% of TP), i.e., vivianite-P (20–25% of TP) and Fe(III)-P (43–45% of TP). The latter was mainly amorphous FePO4 and/or P sorbed to ferric (hydr)oxides as vivianite was the only crystalline P mineral. This study reveals that the P variation in DPS throughout the dairy processing WWTP resembles that in sewage sludge throughout municipal WWTPs. The dominance of Fe-P and biogenic P render vivianite a promising P recovery product from DPS if anaerobic treatment and extra Fe dosing are applied.