Mare Smythii /ˈsmɪθiaɪ/ (Latin for "Smyth's Sea") is a lunar mare located along the equator on the easternmost edge of the Moon's near side, named for the 19th-century British astronomer William Henry Smyth.

The Smythii basin where the mare is located is of the Pre-Nectarian epoch,[2] while the surrounding features are of the Nectarian system.

As a result of this sequence, only the northeastern portion and a smaller area in the western central contain significant regions of smooth mare material.

[5] The geological features suggest that lava flowed from the southeast and west toward the northeast, where it accumulated in the lowest parts of the basin.

This crater makes up part of the southern rim of Mare Marginis.