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Abstract Cutaneous T‐cell lymphomas (CTCLs) are unique non‐Hodgkin lymphomas due to the proliferation of skin‐homing T cells. Sezary syndrome (SS), a subtype of CTCL, typically presents as erythroderma with specific blood involvement and seldomly evolves from tumor‐stage mycosis fungoides (MF). The article examines the case of a 62‐year‐old female patient who developed SS while under atezolizumab treatment for tumor‐stage transformed MF. Postdiagnosis, she underwent treatments including topical corticosteroids, phototherapy, and other drug regimens, experiencing both complete responses and relapses over the years. Three years postdiagnosis, her condition deteriorated with facial tumor lesions, confirmed as MF. Despite multiple treatments, including oral drugs, radiation therapy, and a range of other therapeutic agents, her condition did not show sustained improvement. The patient was then administered atezolizumab, an antibody targeting programmed death ligand 1 (PD‐L1). Although initial results saw a disappearance of her skin tumors, the subsequent cycles led to symptoms consistent with stage IVA1 SS, coupled with an evident shift in malignant T‐cell clone dominance. Atezolizumab, a humanized monoclonal antibody, impedes the interaction of PD‐L1 with its receptor PD1, which is notably overexpressed in malignant T‐cells of SS. This case illustrates the emergence of SS during the course of atezolizumab treatment for a transformed MF condition, underlining the potential risks and complexities in using PD‐L1 inhibitors for treating CTCL.