Sinus Medii ("Central Bay") is a small lunar mare. It takes its name from its location at the intersection of the Moon's equator and prime meridian; as seen from the Earth, this feature is located in the central part of the Moon's near side, and it is the point closest to the Earth. From this spot the Earth would always appear directly overhead, although the planet's position would vary slightly due to libration.
The northern edge of the Sinus Medii is formed by a highland region, with the impact craters Murchison and Pallas along the border. Near the northern border on this mare is the cup-shaped Chladni.
Another highland region lies to the south and southeastern edge of the Sinus Medii. Several flooded craters lie along this border, with Flammarion near the western edge, then Oppolzer, Réaumur, and Seeliger further east. The Rima Flammarion and Rima Oppolzerrilles lie along the edge of the mare near their corresponding craters. Also along the southeast border and bisecting the prime meridian is the crater Rhaeticus.
In the western half of the interior are the small craters Bruce and Blagg. Near the western end of the mare are the flooded craters Schröter and Sömmering.