Polymyositis (PM) is a systemic disease of the idiopathic inflammatory myopathy group, clinically characterized by symmetric and proximal muscle weakness. There are reports in literature of PM associated with malignancies, autoimmune diseases, and viral infections. However, the association between PM and nephropathy is not common. We describe a case report of a patient with polymyositis who developed nephrotic syndrome due to mesangial glomerulonephritis.