Malignant lymphoma developing during anti-PD-1 antibody treatment is extremely rare. A 74-year-old female was admitted with left hypochondrial pain. She was diagnosed with squamous cell carcinoma of the right upper lobe of the lung, and had undergone surgery and postoperative chemotherapy three years prior. Needle biopsy of a mediastinal lymph node revealed recurrent lung cancer (LC). Pembrolizumab (PEM) monotherapy was started as salvage treatment. Although her lymphadenopathy improved, thrombocytopenia and splenomegaly developed during treatment with nine doses of PEM. Laboratory findings included anemia, increased lactate dehydrogenase, and soluble interleukin-2 receptor levels of 6379 U/mL. Flow cytometry of peripheral blood and bone marrow showed CD20, κ ≪  λ cell populations. IGH-BCL2 fusion was detected by fluorescence in situ hybridization in bone marrow. Positron emission tomography showed abnormal uptake in tonsils, both cervical lymph nodes, mediastinum (different location from the recurrent LC), spleen, and abdominal cavity. Follicular lymphoma (FL) grade 1/2 was histologically diagnosed by tonsillar biopsy. She achieved a complete metabolic response (CMR) after rituximab monotherapy on PEM discontinuation. Relapsed FL was diagnosed by submandibular gland biopsy four months after restarting PEM and she achieved a second CMR after rituximab-containing chemotherapy. We describe the first case of newly diagnosed FL during PEM treatment.