The acidic environment within lysosomes is maintained within a narrow pH range (pH 4.5-5.0) optimal for digesting autophagic cargo macromolecules so that the resulting building block metabolites can be reused. This pH homeostasis is a consequence of proton influx produced by a V-type H + -translocating ATPase (V-ATPase) and rapid proton efflux through an unidentified "leak" pathway. By performing a candidate expression screening, we discovered that the TMEM175 gene encodes a proton-activated, proton-selective channel (LyPAP) that is required for lysosomal H + "leak" currents. The activity of LyPAP is most active when lysosomes are hyper-acidified, and cells lacking TMEM175 exhibit lysosomal hyper-acidification and impaired proteolytic degradation, both of which can be restored by optimizing lysosomal pH using pharmacological agents. Variants of TMEM175 that are associated with susceptibility to Parkinson disease (PD) cause a reduction in TMEM175-dependent LyPAP currents and lysosomal hyper-acidification. Hence, our studies not only reveal an essential H + -dissipating pathway in lysosomes, but also provide a molecular target to regulate pH-dependent lysosomal functions and associated pathologies.