Occurrence of early nephrotic syndrome in type 1 diabetes mellitus patients is extremely rare. Herein, we report the case of a 12-year-old boy who presented to our pediatric nephrology clinic with generalized edema. He had been diagnosed with type 1 diabetes mellitus at age 9 and had been treated with regular insulin. Examinations revealed normal kidney function, hypoalbuminemia, proteinuria (4+), hyperlipidemia, and low protein-to-creatinine ratio. The patient was diagnosed with idiopathic nephrotic syndrome and was empirically administered prednisolone for 12 weeks. Subsequently, prednisolone was tapered over 10–12 weeks. The patient showed good response to treatment. In conclusion, co-existence of nephrotic syndrome and type 1 diabetes mellitus may suggest an immunological basis; therefore, further studies are needed to investigate the relationship between these two conditions.