Purpose: The management of bilateral diffuse uveal melanocytic proliferation is controversial, with most earlier reports advocating for plasmapheresis. Here, we report a case of bilateral diffuse uveal melanocytic proliferation secondary to gastric adenocarcinoma that was successfully treated with adjunctive intravenous immunoglobulin, using a loading dose followed by a treat-and-extend strategy. Methods: Clinical case report with 3 years of follow-up including serial ophthalmologic examinations. Functional assessments included visual acuity and subjective visual complaints. Periodic anatomic assessments were performed including serial optical coherence tomography, ultra-widefield fundus imaging, multimodal angiography, and ophthalmic ultrasound. Results: A 75-year-old man presented with a 6-month history of blurry vision and difficulty with dark and light adaptation in both eyes. A diagnosis of bilateral diffuse uveal melanocytic proliferation was established based on clinical findings and subsequent discovery of an underlying gastric malignancy; the diagnosis of a paraneoplastic syndrome was further substantiated by documentation of cancer-associated antiretinal antibodies and serum reactivity against human retina by immunohistochemistry. Treatment with high-dose (2 g/kg) intravenous immunoglobulin was initiated with biweekly, followed by monthly, then q2-3 monthly dosing intervals. Meanwhile, the patient received surgical resection and systemic chemotherapy resulting in complete radiographic remission of the primary cancer. Over the course of 3 years, the patient has demonstrated clinically significant improvements in visual acuity and subjective visual function. Rapid and sustained anatomic improvements were observed in serous retinopathy and choroidal thickness. Conclusion: Intravenous immunoglobulin may be a viable adjunctive treatment option for some patients with bilateral diffuse uveal melanocytic proliferation.