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.