<p>Occult breast cancer (OBC) is a rare and special type of breast cancer. The initial symptom is an enlarged axillary lymph node. Paraneoplastic neurological syndromes (PNS) are a group of syndromes that are caused by malignant tumors and damage to the nervous system in distant parts. Most patients have neurological symptoms before the primary tumors. Early diagnosis and treatment are beneficial to improve the therapeutic effect of patients. This case report describes a 54-year-old woman presenting with a one-year history of gradual-onset abnormal involuntary movements, psychiatric disorders, mental decline, and weight loss. Neurological evaluation showed that delirium, involuntary movements and muscle tension increased. Although extensive diagnostic tests were carried out, including routine and biochemical analyses of cerebrospinal fluid (CSF) and hematological examinations, no obvious abnormalities were detected except that the antinuclear antibody and tissue-based analysis (TBA) in serum and CSF were positive. Initial treatments, including glucocorticoid, antiviral therapy and immunoglobulin, were ineffective. Further systemic ultrasonography and positron emission tomography revealed enlarged hyper-metabolic lymph nodes in the right axillary and bilateral sub mental regions. Pathological examination confirmed adenocarcinoma infiltration, consistent with breast origin, leading to a diagnosis of OBC and PNS. Combination therapy was initiated with palbociclib (CDK4/6 inhibitor) plus letrozole (aromatase inhibitor), which led to the gradual improvement of involuntary movements, although cognitive impairment and indifference still existed. This case highlights the diagnostic challenges of OBC and PNS, emphasizing the importance of comprehensive diagnostic approaches and the potential for neurological symptoms to precede tumor detection. To our knowledge, this is a rare case of OBC with TBA positivity presenting with parkinsonism and cognitive disorder, contributing to the understanding of PNS complexity.</p>