POEMS syndrome is a rare paraneoplastic disorder characterized by polyneuropathy, organomegaly, endocrinopathy, monoclonal plasma cell disorder, and skin changes. Although Castleman disease (CD) is observed in up to 30% of POEMS cases, it is typically of the multicentric subtype (MCD) and identified during the initial diagnostic workup. Unicentric Castleman disease (UCD), by contrast, is rarely associated with POEMS and has not been previously reported following POEMS remission. We present the case of a 39-year-old woman who was diagnosed with POEMS syndrome based on clinical, radiographic, and histopathologic findings, including an IgA lambda-restricted plasma cell neoplasm, sclerotic bone lesions, thrombocytosis, and markedly elevated VEGF levels. She achieved clinical and biochemical remission after 17 cycles of daratumumab, cyclophosphamide, bortezomib, and dexamethasone. Eighteen months later, she developed a tender inguinal lymph node with no systemic signs of relapse. Excisional biopsy revealed histopathologic features consistent with UCD, including IgA lambda-restricted plasma cells, alternating hyperplastic and atretic follicles, and a “lollipop” lesion. To our knowledge, this is the first reported case of UCD arising after remission of POEMS syndrome. The presence of clonally restricted plasma cells within the lymph node, matching the original neoplastic clone, raises the possibility of localized residual disease rather than a de novo process. This case highlights the need for ongoing vigilance in POEMS patients, as late-onset lymphoproliferative disorders such as UCD may emerge in the absence of traditional disease markers or systemic relapse.