Bradyrhizobium diazoefficiens USDA110 is an important soybean symbiont widely used as a nitrogen biofertilizer, but its capacity to solubilize inorganic phosphate remains largely unknown. This study evaluated its ability to solubilize insoluble tricalcium phosphate [Ca₃(PO₄)₂] and examined the effect of pyrroloquinoline quinone (PQQ) on this process. USDA110 solubilized Ca₃(PO₄)₂, increasing dissolved phosphate up to 69.5 ± 3.2 µg PO₄³⁻/ml. Phosphate release was accompanied by a decrease in culture pH from approximately 7.0 to 5.0. Supplementation with 10 µM PQQ significantly enhanced phosphate solubilization, resulting in the accumulation of up to 81.8 ± 4.3 µg PO₄³⁻/ml per 10⁹ cells by day 9. An apo-glucose dehydrogenase reconstitution assay detected extracellular PQQ under all carbon-source and phosphate conditions examined, with PQQ concentrations influenced by both factors. Expression of the pqqB-E genes was generally lower under high-P conditions than under low- and medium-P conditions, although pqqA, the first gene in the pqq operon, remained highly expressed across all conditions. RNA-seq analysis showed no transcriptional induction of genes encoding glucose or glycerol dehydrogenases following PQQ supplementation. However, this finding does not exclude their involvement through the PQQ-dependent activation of pre-existing enzymes. Alternative dehydrogenases, organic acid production, or membrane-associated proton/ion transport may contribute to the enhancement. Hence, the molecular mechanism underlying PQQ-enhanced phosphate solubilization requires further investigation. These findings demonstrate that B. diazoefficiens USDA110 possesses acidification-associated phosphate-solubilizing activity that is enhanced by exogenous PQQ, supporting its potential as a dual-function biofertilizer for nitrogen fixation and P mobilization in soybean cultivation.