Many Patescibacteria lineages are abundant in wastewater treatment plants, yet their interaction partners and ecological roles remain poorly understood. Here, we identify a specific association between a novel Saccharimonadia genus, Candidatus Lutisaccharimonas, and the widespread polyphosphate-accumulating organism (PAO) Candidatus Phosphoribacter. Fluorescence in situ hybridization revealed close physical proximity between the two taxa, supporting an epibiotic relationship. Correlated population dynamics, CRISPR spacer matches and evidence of horizontal gene transfer further corroborated this association. Comparative genomic analyses showed that Ca. Lutisaccharimonas has greater metabolic versatility than is typical of Patescibacteria, including complete pathways for de novo nucleotide biosynthesis and genes involved in amino acid metabolism. These metabolic features may shape its interactions with Ca. Phosphoribacter. Together, our findings identify a previously undescribed host association for Patescibacteria, expand current understanding of their metabolic potential, and suggest that the interactions between Saccharimonadia and PAOs may influence phosphorus removal in engineered ecosystems.