Subpial hematoma is a rare type of intracranial hemorrhage with a poor prognosis. A new mechanism to explain subpial Sylvian hematoma formation in acute post-aneurysmal subarachnoid hemorrhage (SAH) was observed during coiling of a posterior communicating artery aneurysm. Multiple small bleeding points from pial branches of the left middle cerebral artery, which were remote from the ruptured aneurysm, were observed on conventional angiography. This bleeding led to the formation of a large expanding subpial Sylvian hematoma. Similar observations have been recently demonstrated on 4D CT angiography (4D CTA). We present a case that adds evidence to the literature in support of a new mechanism of subpial hematoma formation in the setting of acute SAH. This may advocate the early use of 4D CTA and conveys a poor prognosis, which might influence treatment decisions.