Primary bone lymphoma (PBL) is rare bone disease that accounts for very small number of all primary bone tumors. Among the described sites of PBLs, the patella is an extremely rare example To date, only a few cases of PBL affecting the patella have been reported. Clinically, these tumors have very similar presentation of pain, decreased range of motion and swelling and, sometimes, pathologic fractures. On radiographs, skeletal lymphoma commonly manifests as osteolytic lesions with ill-defined margins affecting the metaphysis of axial long bones. We present a rare case of patellar adult T-cell lymphoma/leukemia in a 58-year-old female who presented with left-knee pain and swelling. Computed tomography and magnetic resonance imaging revealed diffuse marrow replacement by a lesion with aggressive features. PET scan demonstrated neoplastic range hypermetabolic FDG uptake within this lesion. Ultrasound-guided biopsy was consistent with PBL.