Purpose <p>Cerebral arterial fenestration is a common occurrence. However, fenestration of the middle cerebral artery (MCA) is uncommon. It is usually observed in the proximal M1 segment and has a small slit-like configuration. We herein report a case of a relatively large semicircular fenestration located in the mid M1 segment of the right MCA.</p> Methods <p>Magnetic resonance imaging (MRI), including magnetic resonance angiography (MRA), was performed in a 66-year-old man presenting with left homonymous hemianopia for the evaluation of suspected cerebrovascular disease. The MR machine was a 3-Tesla scanner, and MRA was performed using the standard three-dimensional (3D) time-of-flight technique.</p> Results <p>MRA demonstrated fenestration in the mid M1 segment of the right MCA. The fenestration appeared in a relatively large semicircular configuration. The superior limb took a semicircular course and was thinner than the inferior limb, and no perforating branches arising from either limb were clearly identified.</p> Conclusion <p>Fenestration of the MCA is thought to result from incomplete fusion or partial persistence of the arterial plexus during embryological development and is therefore typically small, slit-like, and located in the proximal M1 segment. However, the morphology observed in this case was markedly different. To our knowledge, only a few similar cases have been reported in the English-language literature.</p>