Patients having rheumatoid arthritis (RA) treated with methotrexate (MTX) are at an increased risk of developing lymphoproliferative disorder (LPD). Epstein–Barr virus (EBV) sometimes contributes to the development of MTX-associated LPD. Herein, we report the case of a 64-year-old Japanese woman with RA who showed complications of EBV-positive MTX-associated LPD. This case is exceedingly rare in that the LPD was confined to the lungs and its subclassification was extranodal NK/T-cell lymphoma. Only four cases of extranodal NK/T-cell lymphoma in the setting of MTX-associated LPD have ever been reported in the English language literature, only one of which was an extranasal NK/T-cell lymphoma, similar to our case. Extranasal NK/T-cell lymphomas show more aggressive behavior than nasal NK/T-cell lymphomas, possibly reflected by the considerable re-exacerbation of the lesions in only two months after the cessation of MTX in our case. However, the SMILE regimen (steroid, methotrexate, ifosfamide, l-asparaginase, and etoposide) was able to suppress tumor growth in this case.