We report a unique case of thalamic dementia caused by tentorial dural arteriovenous malformation (AVM), which was successfully treated by surgical occlusion of the straight sinus after attempts to occlude the dural AVM by endovascular procedures had failed. The tentorial dural AVM was fed by bilateral posterior meningeal arteries and occipital arteries, with early retrograde venous drainage into the straight sinus, inferior sagittal sinus, basal vein of Rosenthal, and dural veins in the falx cerebri. The venous ischemia in bilateral thalamus was revealed as low-density areas in computed tomographic scans, as high-intensity areas in T2-weighted magnetic resonance images, and as decreased cerebral blood flow and increased cerebral blood volume in single-photon positron emission computed tomography. Because this patient was not amenable to cure by endovascular embolization, an open surgical approach, surgical occlusion of the draining venous channel (the straight sinus), was performed. Postoperatively, dural AVM in angiography and venous ischemia in bilateral thalamus were resolved, as revealed by magnetic resonance imaging and single-photon positron emission computed tomography. Dementia caused by bilateral thalamic ischemia was also resolved. This case clearly illustrates the pathophysiological mechanism of venous ischemia in the bilateral thalamus to cause reversible dementia.