Introduction <p>Retroperitoneal ganglioneuromas are rare, benign neural crest derived tumours that usually present incidentally as well defined, non-functioning retroperitoneal masses. Moyamoya disease, in contrast, is a progressive occlusive arteriopathy of the distal internal carotid arteries causing ischaemic events in childhood. The coexistence of these two entities has not been previously documented in literature.</p> Case presentation <p>A previously healthy 5-year-old girl presented with acute right-sided weakness and a gradually enlarging, painless left abdominal mass. Brain MRI revealed T2/FLAIR hyperintensity and diffusion restriction in the left frontal and parietal lobes. MR angiography demonstrated severe bilateral supraclinoid internal carotid artery narrowing with basal collaterals, diagnostic of Moyamoya disease. Contrast-enhanced CT abdomen showed a 6.5 cm well-circumscribed, hypoenhancing retroperitoneal mass abutting bowel loops and pancreas, suggestive of a neural origin. Complete surgical excision yielded a solid, encapsulated mass; histopathology revealed mature ganglion cells within schwannian stroma, and immunohistochemistry (Synaptophysin+, NSE+, AMACR+) confirmed retroperitoneal ganglioneuroma. Postoperative recovery was uneventful, and neurological deficits improved under antiplatelet therapy.</p> Discussion <p>Recent literature (2015–2024) documents several paediatric retroperitoneal ganglioneuromas, all with favourable prognosis following complete excision, but none with concurrent Moyamoya disease. The co-occurrence in this child may reflect shared developmental pathways of neural-crest derived cells and vascular smooth muscle. The case underscores the need for comprehensive neuroimaging when neurological symptoms accompany a benign appearing abdominal mass.</p> Conclusion <p>This case highlights the importance of multidisciplinary evaluation and coordinated management when dual pathologies are identified. </p>