Pyoderma gangrenosum (PG) is a destructive inflammatory disease characterized by painful ulcerative lesions with neutrophilic infiltration in the dermis. Although the etiology and pathogenesis are poorly understood, uncontrolled inflammatory responses to nonspecific stimuli may trigger the development of PG. Here, we report a rare case of PG arising above a venous insufficiency lesion. The most typical diagnosis for an ulcer at such a location is venous ulcer. However, our atypical case of PG emphasizes that PG can be induced by ischemic conditions, such as venous insufficiency. Debridement and skin grafting, which were performed based on an incorrect diagnosis, worsened the PG lesion and provoked a severe pathergy phenomenon. Thus, surgeons should consider PG in the differential diagnosis of intractable venous ulcers in order to ensure an accurate diagnosis and prompt treatment.