The TNF-α antagonists are the drugs used for the treatment of ulcerative colitis (UC). Nontraumatic convexity subarachnoid hemorrhage is an infrequent nonaneurysmal subtype of subarachnoid bleeding caused mainly by reversible cerebral vasoconstriction syndrome (RCVS), cerebral amyloid angiopathy, and posterior reversible encephalopathy syndrome (PRES). We present a 26-year-old female patient with a diagnosis of UC taking Adalimumab. She received her last doses the same day she was admitted to our hospital for an acute severe UC exacerbation. Steroids were added to the treatment. Five days after admission she presented a thunderclap headache with photophobia, nausea, and vomiting. An MRI was performed showing left frontal convexity subarachnoid hemorrhage and hyperintense lesions on T2-weighted and FLAIR sequences located in both occipital lobes, left cerebellar hemisphere, and brainstem. Digital angiography was unremarkable. Adalimumab was discontinued but persisted on treatment with steroids. The patient evolved with complete resolution of her symptoms and was discharged with a normal neurological exam. Two months later, she was asymptomatic and her MRI revealed superficial siderosis secondary to cSAH with resolution of white matter hyperintensities. Convexity subarachnoid hemorrhage in our patient could be secondary to PRES or to RCVS. Analogous MRI findings can be observed in both syndromes, along with similar clinical and angiographic findings. This suggests that both conditions may reflect different manifestations of the same pathology, in which vascular tone and endothelial dysfunction play a major role. To our knowledge, this is the first report of a patient with severe UC and convexity subarachnoid hemorrhage associated with Adalimumab.