The present study examined the P and K recovery from biomass bottom ash of a power plant that uses agricultural and animal wastes as fuel. The extractability of the elements contained in the biomass bottom ash was investigated using various organic and inorganic acids. H2SO4 yielded the most extracted P, K, and Mg. The optimum experimental conditions were determined using the Box–Behnken experimental design at 0.6 M H2SO4, 25 °C, and 8/1 liquid/solid ratio. A crystallization process was applied to recover P, K, and Mg from the obtained extracted liquid under optimum conditions. At a Mg:K:P molar ratio of 1:1:1 and pH 9, removal efficiencies of PO4-P, K, and Mg were 98.67%, 47.44%, and 96.81%, respectively. The most efficient removal of PO4-P and K was at pH 9 and a Mg:K:P molar ratio of 1.8:1:1.5. The crystal structure, morphology, and chemical composition of the precipitate recovered under these conditions were determined using XRD and SEM–EDS analysis. The results showed that hazenite (Mg2KNa(PO4)2·14H2O) and K-struvite (MgKPO4·6H2O) crystals were present in the precipitate. The study revealed that P, K, and Mg in biomass bottom ash could be recovered in the forms of hazenite and K-struvite, which have fertilizer properties.