Introduction: Ocular immune-related adverse events (OirAEs) associated with novel cancer therapies of immune checkpoint inhibitors (ICIs) are emerging. Retrobulbar optic neuritis (ON) combined with optic perineuritis (OPN), associated with atezolizumab, has been rarely reported and has a unique clinical manifestation. Case Description: A 67-year-old woman was diagnosed with small-cell lung cancer. As maintenance therapy, atezolizumab was administered continuously for 10 cycles for approximately 14 months. One week after the administration of the tenth dose of atezolizumab, the patient experienced a bilateral, successive painless visual decline. The fundus and the retinal nerve fiber layer revealed no abnormalities, but the ganglion cell of the macula disappeared loss. The concentric shrinking of the peripheral visual field of the left eye was noticed. Orbital MRI revealed bilateral optic nerve thickening and peripheral optic nerve sheath enhancement ("tram-track" and "doughnut" signs). Serology, cerebrospinal fluid results, and image examination ruled out common causes of vision decline, and the condition was identified as bilateral retrobulbar ON combined with OPN as a probable atezolizumab-related immune adverse event. Thereafter, atezolizumab was discontinued, and intravenous methylprednisolone pulse (IVMP) (160 mg/day for 5 days) plus intravenous immunoglobulin (20 g/day for 3 days) was administered. The patient's visual function considerably improved after three weeks. Conclusions: Retrobulbar ON and OPN associated with atezolizumab are rare side effects that are easily overlooked. Immune-related ON has unique features and requires early identification. The primary treatment for optic nerve irAEs is corticosteroids, but this is not standardized and should be used with caution.