Foreign body reactions after endovascular procedures for brain arteriovenous malformations are extremely rare. We report the case of a cerebral foreign body granuloma reaction after embolization of a frontal arteriovenous malformation with Onyx. A previously treated 36-year-old man underwent re-embolization of a residual and recurrent unruptured right frontal vascular malformation with Onyx. The post-procedural imaging revealed a right frontotemporal heterogeneously enhancing expansive lesion associated with a residual malformation. Following microsurgical resection, the histopathological examination of the expansive lesion revealed basophilic foreign body like deposits adjacent to multi-nucleated giant cells, highly compatible with cerebral foreign body granulomas reaction to Onyx. The clinical and radiological follow-up of the patient was favorable after complete resection of the lesions.