Abstract: Sludge incineration is applied in wastewater treatment plants for sludge volume reduction and sanitization. As sewage sludge is a promising secondary source for phosphorus (P), it is crucial to understand the transformation of P speciation during sludge incineration, which can provide fundamental insights for P recovery from the resulted sludge ash. This study examined the P speciation in a dewatered digested sludge and its sludge ash after incineration (800 °C), which were collected from a local municipal wastewater treatment plant. The results showed that the incineration tripled the total P (TP) content in the sample (from 25 to 75 mg/g). Organophosphate, which accounted for 7% of TP in the dewatered digested sludge, was completely converted to inorganic orthophosphate after incineration. This resulted in the dominance of inorganic orthophosphate (~99% of TP) in the sludge ash. P fractionation revealed a significant transformation from iron-bound P (Fe-P, 70% of TP in the dewatered digested sludge) to calcium-bound P (Ca-P, 80% of TP in the sludge ash) under incineration. This implies that acidic extraction could be an efficient approach for P recovery from sludge ash, given the easy dissolution of Ca-P in acidic conditions. The findings provide valuable guidance for the P recovery practice from sludge ash.