Background: Variants in TBC1D8B cause nephrotic syndrome. TBC1D8B is a GTPase-activating protein for Rab11 (RAB11-GAP) that interacts with nephrin,,but how it controls nephrin trafficking or other podocyte functions remains unclear. Methods: We generated a stable deletion in TBC1D8B using microhomology-mediated end joining genome editing. Ex vivo functional assays utilized slit diaphragms in podocyte-like Drosophila nephrocytes. Manipulated endocytic regulators in transgenic mice provided a comprehensive functional analysis of TBC1D8B . . Results: A null allele of Drosophila TBC1D8B exhibited nephrocyte-restricted nephrin mislocalization, similar to patients with isolated nephrotic syndrome who have variants in the gene.. The protein was required for rapid nephrin turnover in nephrocytes and for endocytosis of nephrin induced by excessive Rab5 activity. The protein expressed from TBC1D8B bearing the edited deletion predominantly localized to mature early endosomes and late endosomes and was required for endocytic cargo processing and degradation. Silencing Hrs, a regulator of endosomal maturation, phenocopied loss of TBC1D8B. Low-level expression of murine TBC1D8B rescued loss of the Drosophila gene, indicating evolutionary conservation. Excessive murine TBC1D8B selectively disturbed nephrin dynamics. Finally, we discovered four novel TBC1D8B variants within a cohort of 363 FSGS patients and validated functional impact of two variants in Drosophila, suggesting a personalized platform for TBC1D8B-associated FSGS. Conclusion: Variants in TBC1D8B are not infrequent among FSGS patients. TBC1D8B, functioning in endosomal maturation and degradation, is essential for nephrin trafficking.