We describe the electroretinographic findings of a case of primary intraocular lymphoma (PIOL) wherein the patient received intravitreal injections of methotrexate (ivMTX). A 62-year-old man developed blurred vision and complained of decreased visual acuity (VA) in his right eye. Fundus examination showed vitreous opacity and multiple subretinal yellowish lesions. Optical coherence tomography (OCT) revealed subretinal and intraretinal infiltrations. The full-field electroretinogram (ffERG) showed subnormal combined rod-cone response and multifocal electroretinogram (mfERG) recorded using skin electrodes showed severe attenuation of the response compared with the other eye. Pars prana vitrectomy, phacoemulsification, and lens implantation were performed to remove the opacity, and vitreous biopsy revealed a high ratio of interleukin 10–6 (76.0). There was no systemic malignant lesion, and the patient was diagnosed with PIOL. Treatment with ivMTX (400 μg/0.1 mL) was started. One month later, the intraretinal infiltration had disappeared, and mfERG revealed recovery of the response density from the central area. Two months later, OCT showed recovery of the foveal ellipsoid and interdigitation zones, and VA recovered to 20/17; mfERG showed maintenance of macular function. However, the amplitude of a- and b-waves in the ffERG gradually decreased. Macular function recovered, but there was also a decrease in total retinal function. mfERG and ffERG recorded using skin electrodes were useful in monitoring macular and entire retinal function with repeated examinations and showed recovery and maintenance of macular function in a case of PIOL treated with ivMTX.