Importance Immune-mediated inflammatory diseases are rare but increasingly reported among patients with myelodysplastic syndromes (MDS) or chronic myelomonocytic leukemia (CMML). Systemic lupus erythematosus (LE) and cutaneous LE associated with MDS/CMML have been rarely described, with atypical features and refractory disease. Objective To provide a comprehensive description of the phenotype and therapeutic responses of LE associated with MDS/CMML and to compare them with idiopathic LE. Design, Setting, and Participants This retrospective case-control study included nationwide, multicenter data from January 1975 to January 2023. Patients with MDS/CMML who either fulfilled classification criteria for systemic LE or had skin lesions diagnosed as cutaneous LE were included. For MDS/CMML systemic LE, a 2:1 case-control study was conducted with idiopathic systemic LE. Clinical features, centralized skin histopathology, and targeted next-generation sequencing were analyzed. Data were analyzed from May 2022 to June 2025. Main Outcomes and Measures The clinical, pathological, and molecular features of LE occurring in the setting of MDS or CMML compared with idiopathic LE. Results Of 24 included patients, 9 (38%) were female, 15 (63%) were male, and the median (range) age at diagnosis was 65 (32-85) years. A total of 19 were diagnosed with systemic LE and 5 with cutaneous LE. The median (range) follow-up was 4.5 (1-31) years. Cutaneous involvement was the most common manifestation of LE (17 [71%]). Chilblain lupus was the predominant subtype (6 [35%]). Compared with idiopathic systemic LE, patients with MDS/CMML–associated LE were older (median [range] age, 65 [32-85] years vs 23 [11-55] years; P &lt; .001), more frequently male (10 [53%] vs 3 [8%]; P = .008), had less kidney involvement (2 [10%] vs 27 [71%]; P &lt; .001), had less articular involvement (7 [36%] vs 37 [97%]; P &lt; .001), and had reduced anti–double-stranded DNA positivity (6 [32%] vs 29 [76%]; P = .001). The underlying hematologic diseases included MDS (16 [66%]) and CMML (8 [34%]), with 22 (92%) classified as lower risk (Revised International Prognostic Scoring System score of 3.5 or less). Centralized histopathological review reclassified 6 skin biopsies (50%) as MDS/CMML cutis. Identical myeloid variants were detected in blood and skin in 6 of 8 patients, supporting a clonal inflammatory process. Standard LE therapies were often poorly effective, while clone-directed therapies (azacitidine or allogeneic hematopoietic stem cell transplant) led to parallel hematologic and LE responses in 5 of 7 patients. Conclusions and Relevance In this study, MDS/CMML–associated lupuslike manifestations were a distinct entity mimicking systemic LE or cutaneous LE and characterized by clonal inflammation rather than classic autoimmunity in most cases. Early recognition is important, as treatment may require clone-targeting therapies rather than conventional LE therapy.