Abstract: Background　Mucosa-associated lymphoid tissue-type diffuse large B-cell lymphoma (MALT-DLBCL) of the thyroid gland is a rare special subtype of primary thyroid lymphoma (PTL). This subtype progresses and transforms from extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT), and its biological behavior is different from that of simple MALT lymphoma or diffuse large B-cell lymphoma (DLBCL), with no unified standards for clinical diagnosis and treatment. Objective　To analyze the clinical features and treatment process of one case of MALT-DLBCL, with the aim of further clarifying the diagnostic and therapeutic strategies for primary thyroid MALT-type transformed diffuse large B-cell lymphoma, thereby improving our understanding of this disease. Methods　The clinical manifestations, laboratory test results, imaging features, pathological diagnosis, as well as the treatment course and follow-up information of one MALT-DLBCL patient were systematically collected and organized after being admitted to the Fifth Medical Center of PLA General Hospital on October 30, 2024. Results　 The patient was a 54-year-old female who presented with "diffuse enlargement of the thyroid gland for more than 7 years, with local increase in size over the past 2 months. " Both thyroid ultrasound and PET‑CT suggested a high likelihood of malignancy for the nodular thyroid lesion. Fine-needle aspiration biopsy of the thyroid mass yielded a pathological diagnosis of extranodal mucosa‑associated lymphoid tissue marginal zone lymphoma with transformation to diffuse large B-cell lymphoma. The patient was then treated with four cycles of the R‑CHOP regimen (rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone). After 2 and 4 cycles of chemotherapy, re-examination by PET-CT was assessed as complete metabolic response (CMR); subsequent radiotherapy was delivered to the thyroid gland and cervical lymph node drainage area at a dose of 40 Gy/20 fractions (DT: 40 Gy/ 20 f). During 6 months of follow-up after treatment (as of July 7, 2025), the patient had no discomfort symptoms, and reexaminations including thyroid function, cervical ultrasound and whole-body PET-CT showed no tumor recurrence or metastasis, with a good general condition. Conclusion　The patient with primary thyroid MALT-DLBCL in this case has received a comprehensive treatment regimen of R-CHOP chemotherapy combined with local radiotherapy, achieving complete metabolic response without recurrence during short-term follow-up, indicating that this integrated approach may be appropriate for patients with this rare subtype. Given the low incidence of this subtype and the lack of unified diagnostic and therapeutic standards, individualized plans should be formulated based on the patient's pathological characteristics and comorbidities in clinical practice. Meanwhile, long-term close follow-up is necessary to further clarify its long-term efficacy and prognostic features, thereby accumulating more case experience for clinical diagnosis and treatment.