Immunoglobulin G4-related retroperitoneal fibrosis (IgG4-RPF) is a rare fibroinflammatory condition characterized by the infiltration of IgG4+ plasma cells and fibrosis, affecting the retroperitoneum and causing obstructive uropathy. We report a rare case of a 32-year-old male who experienced persistent left lumbar pain for 1.5 years. Imaging revealed a para-aortic soft tissue mass encasing the proximal ureter, causing mild hydronephrosis. Histological examination confirmed IgG4-RPF, indicated by marked fibrosis and plasma cell-rich inflammatory infiltrates. The patient received corticosteroid therapy followed by rituximab, resulting in significant clinical improvement and mass regression. This case highlights the need for awareness and understanding of IgG4-RPF as a distinct entity to ensure timely and accurate diagnosis. Early intervention with targeted therapy can effectively manage the condition, prevent severe complications, and improve patient outcomes. Long-term follow-up and monitoring of disease activity are essential to prevent recurrence and manage potential complications associated with IgG4-RPF.