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Objective Endoscopic third ventriculostomy is a well-established procedure for the treatment of obstructive hydrocephalus, restoring cerebrospinal fluid (CSF) circulation by creating a communication between the third ventricle and the prepontine cistern. Conventional approaches rely on anterograde transventricular access, which necessitates cortical and subcortical traversal and carries risks of parenchymal and hypothalamic injury. We present a novel, image-guided neurointerventional technique for performing third ventriculostomy via a retrograde subarachnoid route. This approach builds on previously described subarachnoid catheterization of the ventricular system and extends its application to CSF diversion.Materials and Methods Three formalin-fixed, silicone-injected, alcohol-preserved human cadaveric heads were studied. Subarachnoid access was obtained via direct percutaneous puncture of the cisterna magna. Using 3D cone-beam CT (DynaCT) guidance combined with biplane fluoroscopy, a microcatheter-microwire system was advanced through the cisterna magna, foramen of Magendie, fourth ventricle, and cerebral aqueduct into the third ventricle.After achieving stable positioning at the third ventricular floor, targeted perforation of the tuber cinereum was performed using a microwire, followed by controlled balloon dilation to create a ventriculostomy. Technical success and anatomical accuracy were assessed with real-time imaging and confirmed via transcallosal endoscopic visualization.Results Retrograde catheterization of the third ventricle was achieved in all specimens without transgression of cortical brain parenchyma. In each case, the catheter system was positioned at the ventricular floor with sufficient stability to allow controlled perforation and dilation. Successful creation of a ventriculostomy was confirmed radiographically and endoscopically, demonstrating appropriate localization between the infundibular recess and mammillary bodies with communication into the prepontine cistern. No off-target perforation, false passage, or structural injury was observed. The technique was reproducible and allowed precise execution using standard neurointerventional instruments.Conclusion This study demonstrates the feasibility of a retrograde, subarachnoid, image-guided approach to third ventriculostomy performed entirely through native CSF pathways. By avoiding transcranial and transventricular access, this technique represents a distinct procedural paradigm for CSF diversion.These findings support the potential expansion of neurointerventional approaches in the CSF space in the management of hydrocephalus and related CSF flow disorders. Further in vivo studies are required to evaluate safety, durability, and clinical applicability.Disclosures S. Rahmanov: None. M. Sabahi: None. E. Rahmanova: None. H. Borghei-Razavi: None. B. Adada: None. M. Obrzut: None.