Pyoderma gangrenosum (PG), an ulcerating skin condition, is rare in patients with ulcerative colitis (UC). We report a case of successful treatment of PG in a patient with UC using hyperbaric oxygen therapy (HBOT). The patient had UC that was in remission following treatment with mesalazine and azathioprine therapy. After visiting an orthopedic clinic, the patient opted for treatment with antibiotics and daily dressing of the ulcerative skin lesions, while azathioprine was discontinued. However, the lesions did not improve. Two months later, the patient visited a dermatologist who diagnosed the lesions as PG, and he was admitted to our unit. Surgical debridement and HBOT were performed by a plastic surgeon in the emergency department. After 3 months of HBOT and topical treatment, the patient's PG completely resolved. His UC was still in remission with mesalazine alone. HBOT may be an effective and safe alternative treatment for PG associated with UC, particularly in patients in whom anti-tumor necrosis factor agents are unnecessary.