Membranous nephropathy (MN) is an uncommon cause of nephrotic syndrome in children, accounting for fewer than 5% of cases. Primary MN mediated by antibodies against the phospholipase A2 receptor (PLA2R) is particularly rare in the paediatric population. We report the case of a 14-year-old girl with type 1 diabetes mellitus and autoimmune hypothyroidism who presented with nephrotic syndrome and was subsequently diagnosed with PLA2R-positive primary MN. She presented with periorbital oedema, abdominal distension, and bilateral pitting pedal oedema. Laboratory investigations demonstrated severe hypoalbuminemia (serum albumin 1.2 g/dL), nephrotic-range proteinuria (24-hour urine protein excretion of 3.5 g/day), and preserved renal function (serum creatinine 0.3 mg/dL). Renal biopsy findings, supported by immunohistochemistry, confirmed PLA2R positivity. The patient was treated with four weekly doses of rituximab (375 mg/m2), resulting in complete clinical remission at 6 months. Serum albumin normalised to 4 g/dL, 24-hour urine protein excretion decreased to 250 mg/day, and serum PLA2R antibody levels fell below the normal range. This case highlights the importance of considering PLA2R-mediated MN in adolescents with nephrotic syndrome and the presence of multiple autoimmune comorbidities. It also underscores the diagnostic value of tissue PLA2R testing and demonstrates the efficacy of early rituximab-based B-cell-directed therapy in achieving complete remission while avoiding corticosteroid exposure.