Immune checkpoint inhibitor (ICI)-related acute pancreatitis (irAP) is a rare, potentially life-threatening immune-related adverse event. Whereas CT and MRI remain first-line diagnostic imaging modalities, more patients are presenting with atypical irAP as ICI use increases. To appropriately manage these events, it is important to catalog these presentations and provide comprehensive clinical, radiological, and pathological descriptions to guide evidence-based practice. Here, we present the case of a 66-year-old man with advanced lung adenocarcinoma who, after the fifth course of toripalimab, developed epigastric discomfort and elevated serum amylase and lipase. irAP was suspected, but MRI revealed atypical, multifocal pancreatic lesions. To exclude metastases, an endoscopic ultrasound-guided fine-needle biopsy (EUS-FNB) was performed. EUS revealed a slightly swollen pancreas with heterogeneous echoic signals and scattered hyperechoic areas in the parenchyma without an obvious mass. Histopathological examination of the FNB revealed retention of the normal lobular pancreatic architecture with focal acinar atrophy associated with a CD8+ T lymphocyte-predominant infiltrate, further confirming the diagnosis of irAP. After starting glucocorticoids, his symptoms resolved, serum amylase and lipase rapidly decreased to normal, and the abnormal MRI features diminished. irAP can, therefore, present as multifocal lesions on MRI, and, when metastatic disease requires exclusion, EUS-FNB is an effective way to establish a definitive diagnosis. Refining the histopathological and immunopathological criteria for the diagnosis of irAP is now warranted.