Since its introduction in the early 1990s, endoscopic ultrasound-assisted fine-needle aspiration (EUS-FNA) has been used for sampling of extraintestinal mass lesions and peri-intestinal lymphadenopathy. Although EUS-FNA is highly accurate, lymphomas can be challenging to diagnose using EUS-FNA. We present the case of a 60-year-old male who had experienced upper abdominal discomfort for 1 month. Computerized tomography (CT) examination revealed multiple soft-tissue shadows located above the pancreatic body. The biggest shadow had a cross-sectional area of 7.7 cm × 7.2 cm. Positron emission tomography-CT (PET-CT) imaging showed increased uptake of (18)F-FDG by these soft-tissue shadows. To investigate further, EUS was performed and it revealed the presence of multiple hypoechoic round lymph nodes. During the procedure, EUS-FNA was performed, but only a few dyskaryotic cells were observed by cytological evaluation. EUS-assisted retroperitoneoscopy and lymph node biopsy were performed to obtain more tissue for immunohistochemical analysis and subclassification of lymphoma. Finally, the patient was diagnosed with non-Hodgkin lymphoma, germinal center B-cell-like diffuse large B-cell lymphoma by this technique. EUS-assisted transendoscopic retroperitoneal lymph node biopsy is an alternative procedure for the diagnosis of lymphomas.