Endosomal Na(+), K(+)/H(+) antiporter. Mediates the electroneutral exchange of endosomal luminal H(+) for a cytosolic Na(+) or K(+) (Probable). By facilitating proton efflux, SLC9A9 counteracts the acidity generated by vacuolar (V)-ATPase, thereby limiting luminal acidification. Regulates organellar pH and consequently, e.g., endosome maturation and endocytic trafficking of plasma membrane receptors and neurotransporters (PubMed:15522866, PubMed:24065030, PubMed:28130443). Promotes the recycling of transferrin receptors back to the cell surface to facilitate additional iron uptake in the brain (PubMed:28130443). Regulates synaptic transmission by regulating the luminal pH of axonal endosomes (By similarity). Regulates phagosome lumenal pH, thus affecting phagosome maturation, and consequently, microbicidal activity in macrophages (By similarity). Can also be active at the cell surface of specialized cells, e.g., in the inner ear hair bundles uses the high K(+) of the endolymph to regulate intracellular pH (By similarity). {ECO:0000250|UniProtKB:Q8BZ00, ECO:0000269|PubMed:15522866, ECO:0000269|PubMed:24065030, ECO:0000269|PubMed:28130443, ECO:0000305|PubMed:15522866}. This is the function of Ensembl gene identifier ENSG00000181804 (SLC9A9, solute carrier family 9 member A9).