Background: Primary thyroid lymphoma (PTL) is an uncommon condition, accounting for under 5% of thyroid cancers and no more than 2.5% of all lymphoma cases. Case presentation: We present the case of a 50-year-old gentleman who presented with a rapidly progressive right solitary thyroid nodule and a palpable cervical lymph node. Ultrasound imaging revealed a heterogeneous right TIRADS-4 lesion, and positron emission tomography-computed tomography (PET-CT) showed a fluorodeoxyglucose (FDG) avid thyroid lesion and supradiaphragmatic lymph nodes. Fine-needle aspiration cytology (FNAC) of the thyroid was suggestive of lymphoma, and a cervical lymph node biopsy confirmed diffuse large B-cell lymphoma (DLBCL). The patient was treated with chemotherapy (R-CHOP regimen: Rituximab, Cyclophosphamide, Doxorubicin, Vincristine, Prednisolone) and achieved a complete response. Discussion: Primary thyroid lymphoma is characterized by the presence of lymphoma solely within the thyroid gland or in the thyroid and nearby lymph nodes, without any contiguous spread or metastasis at the time of diagnosis. The most prevalent form of PTL is DLBCL. Diagnosis typically relies on biopsy, as FNAC is not adequate. Staging involves the use of FDG PET-CT and follows the Ann Arbor classification system. The main treatment approach combines chemotherapy (R-CHOP) with radiotherapy, while surgical intervention is considered only for cases that do not respond to chemotherapy or have remaining thyroid disease. The 5-year survival rate stands at 90% for patients with disease confined to the thyroid, but this drops to 35% for those with disease extending beyond the thyroid. Conclusion: Primary thyroid lymphoma is a rare malignancy of the thyroid gland with a challenging preoperative diagnosis. A biopsy is essential for diagnosis, and staging is performed using PET-CT. Treatment primarily involves chemotherapy (R-CHOP) and radiotherapy, with surgery reserved for resistant or residual disease. Take-home message: Primary thyroid lymphoma is a rare thyroid cancer, primarily non-Hodgkin's lymphoma, most commonly DLBCL. Diagnosis is confirmed by biopsy rather than FNAC, and staging is done using PET-CT. The primary treatment involves chemotherapy (R-CHOP) and radiotherapy, not surgery.