ANCA-associated vasculitis (AAV) are characterized by necrotizing inflammation of small- and medium-sized blood vessels, frequently associated with antibodies against myeloperoxidase (MPO) or proteinase 3. In contrast, IgG4-related disease (IgG4-RD) is an immune-mediated fibroinflammatory condition that can affect virtually any organ and is histologically characterized by lymphoplasmacytic infiltrates enriched in IgG4-positive plasma cells. Both conditions may present with localized or systemic disease and share clinical and histopathological features, particularly in the head and neck region, making differential diagnosis challenging. We report the case of an 11-year-old girl who initially presented with bilateral orbital inflammation. Orbital biopsy demonstrated lymphoplasmacytic infiltrates, an elevated IgG4+/IgG+ plasma cell ratio, and obliterative phlebitis, suggestive of IgG4-RD. However, markedly elevated MPO-ANCA levels raised suspicion for AAV. Although glucocorticoid therapy initially achieved clinical improvement, relapses occurred, prompting treatment with rituximab. Subsequently, the patient developed rhinosinusitis, and nasal biopsy revealed lymphoplasmacytic inflammation with multinucleated giant cells, consistent with granulomatosis with polyangiitis (GPA). Treatment was intensified with mycophenolate mofetil and corticosteroids, resulting in remission. This case highlights the diagnostic and therapeutic challenges posed by overlapping features of AAV and IgG4-RD.