Sézary syndrome (SS) is a rare type of cutaneous T-cell lymphoma (CTCL) that is characterized by erythroderma, lymphadenopathy and circulating clonal T-cells (Sézary cells). However, to our knowledge, reactive Langerhans cell (LC) proliferation mimicking Langerhans cell histiocytosis (LCH) associated with SS has not been reported. In this report, we describe an unusual case of reactive LC proliferation mimicking LCH associated with SS in a 57-year-old female patient. With complaints of recurrent skin symptoms and enlarged lymph nodes (LNs), she was admitted to our center with a presumptive diagnosis of LCH as demonstrated by LN biopsy pathology. However, other than adenopathy, no lesions were noted in any organ system commonly involved in LCH. Typical Sézary cells were identified through morphology and further confirmed by flow cytometric immunophenotyping in peripheral blood (PB) and bone marrow (BM). In addition, T-cell receptor gene rearrangement was positive, whereas the BRAF V600E gene mutation was negative in skin, LN, PB and BM. The patient was ultimately diagnosed with SS with reactive LC proliferation. This case should remind clinicians to be wary of diagnosing LCH if LCH-like pathology occurs exclusively in LNs. Moreover, morphologic, immunologic, cytogenetic and molecular biologic studies should be performed to avoid misdiagnosis.