A 24-year-old man presented with abdominal pain and bloody diarrhea. Computed tomography (CT) demonstrated a diffusely enlarged pancreas with increased attenuation of the surrounding peripancreatic fat. Endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) was performed on the hypoechoic area in the pancreatic head, and a histological examination of an EUS-FNA specimen revealed marked eosinophilic infiltration (≥20/HPF). In addition, colonoscopy (CS) revealed redness, erosion, and edema extending from the cecum to the ascending colon, and a CS biopsy revealed marked eosinophilic infiltration (≥20/HPF). The patient was thus diagnosed with simultaneous eosinophilic pancreatitis (EP) and eosinophilic colitis (EC) and was treated with prednisolone. His symptoms improved within a few days, and CT performed two weeks later showed improvement in the diffusely enlarged pancreas, allowing prednisolone to be gradually tapered. The present case demonstrates the importance of pathological examinations in the differential diagnosis of EP and its likely complications (e.g., EC in this case).