Objectives: An attentive effort to correctly classify patients with focal segmental glomerulosclerosis (FSGS) is essential for sound treatment decisions in individual patients as proposed by the KDIGO 2021 guideline for glomerular disease. Methods: We retrospectively gathered and analyzed patients diagnosed with biopsy-proven FSGS in 3 tertiary hospitals. FSGS was classified according to the KDIGO 2021 guideline by two independent nephrologists as follows: primary, genetic, secondary, and undetermined cause (UC). Clinical manifestations and kidney outcomes were explored according to the KDIGO FSGS classification.  Results: Overall, a total of 299 patients were included. The mean age was 55.0 ± 16.8 years old and 61.9% were male. The number of patients categorized into primary, genetic, secondary, and UC was 44 (14.7%), 21 (7.0%), 143 (47.8%), and 91 (30.4%), respectively. Primary FSGS patients showed the highest proportion of having nephrotic syndrome represented by lower serum albumin levels and more proteinuria amount. In addition, they tended to have more diffuse foot process effacement in electron microscopic examinations. Secondary FSGS patients tended to have more hypertension and obesity than patients with other FSGS types. Genetic FSGS patients were youngest (40.4 ± 12.6 years) and had a higher proportion of females than males different from other subtypes. Among genetic FSGS, only 6 patients underwent genetic testing. Although the proportion of patients receiving corticosteroid treatment was highest in primary (97.7%) FSGS, not inconsiderable genetic (23.8%), UC (22.0%), and secondary (14.7%) FSGS patients also received corticosteroid treatments. During 3.1 ± 2.8 years of follow-up, 41(13.7%) had progressed to end-stage kidney disease without showing any difference according to the FSGS subtypes. Conclusions: The new FSGS classification proposed by KDIGO 2021 guideline for glomerular diseases showed important gaps in knowledge and clinical practice. More resources and clinical evidence should be supported to clarify the clinical significance of the new system.