Background: Paraneoplastic cerebellar degeneration (PCD), which displays ataxia and other cerebellar symptoms, is the most common paraneoplastic neurological syndrome (PNS). PCD is more likely to occur in individuals with small cell lung cancer (SCLC), gynecological malignancies, and Hodgkin disease, but it is rarely associated with non-Hodgkin lymphoma (NHL).Case Description: We report a case of PCD accompanying high-grade B-cell lymphoma embedded in an individual's stomach and duodenum, who also presented with acute onset of gait ataxia and slurred speech. The results of the common laboratory tests for neurological disorders, including the paraneoplastic antibody test, were negative. The key to the accurate diagnosis was the positron emission tomography/computed tomography findings. The final diagnosis of high-grade B-cell lymphoma was unclear until the performance of repeated esophagogastroscopy with multipoint deep excavation biopsies. After standard chemotherapy, the patient's gastric tumor was significantly alleviated and cerebellar syndrome was significantly improved.Conclusions: This case highlights the challenges of diagnosing PNS associated with occult malignancy. PNS patients may present with a variety of neurological disorders; Thus, if any unexplained neurological symptoms appear after a series of specific laboratory and imaging tests, a diagnosis of PNS should be taken into consideration in the differential diagnosis list, as it may help clinicians identify asymptomatic malignancies and ensure patients receive correct treatments in a timely manner. A high-quality endoscopic biopsy is essential, as it helps hematologists make an accurate diagnosis of lymphoma with gastroduodenal involvement based on pathology.