A map of Mare Serenitatis. The colored arrows indicate the landing places of Luna 21, Apollo 15, and Apollo 17.

Mare Serenitatis (the "sea of serenity") is a lunar mare that sits just to the east of Mare Imbrium. It is located within the Serenitatis basin, which is of the Nectarian epoch. The material surrounding the mare is of the Lower Imbrian epoch, while the mare material is of the Upper Imbrian epoch. The mare basalt covers a majority of the basin and overflows into Lacus Somniorum to the northeast. The most noticeable feature is the crater Posidonius on the northeast rim of the mare. The ring feature to the west of the mare is indistinct, except for Montes Haemus. Mare Serenitatis connects with Mare Tranquillitatis to the southeast and borders Mare Vaporum to the southwest.

Both Luna 21 and Apollo 17 landed near the east border of Mare Serenitatis.

Some people believe Mare Serenitatis is one of the eyes for the Man in the Moon.