Mare Smythii (sea of William Henry Smyth) is a lunar mare located along the equator on the easternmost edge of the lunar near side. The Smythii basin where the mare is located is of the Pre-Nectarian epoch, while the surrounding features are of the Nectarian system. The mare material, which make up the floor of the mare, is a high alumious basalt, and consists of Upper Imbrian basalt covered by Eratosthenian basalt.

The crater Neper is located to the north of the mare. This crater makes up part of the southern rim of Mare Marginis. Just off to the northwest of the mare are the craters Schubert and Schubert B . The dark mare-filled crater at the southern edge of Smythii is the crater Kästner .