Mare Serenitatis is a lunar mare located to the east of Mare Imbrium on the Moon. Its diameter is 674 km (419 mi).

Mare Serenitatis 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. Mare Serenitatis is an example of a mascon, an anomalous gravitational region on the moon.

A mass concentration (mascon), or gravitational high, was identified in the center of Mare Serenitatis from Doppler tracking of the five Lunar Orbiter spacecraft in 1968. The mascon was confirmed and mapped at higher resolution with later orbiters such as Lunar Prospector and GRAIL.

Like most of the other maria on the Moon, Mare Serenitatis was named by Giovanni Riccioli, whose 1651 nomenclature system has become standardized. Previously, William Gilbert had cited it as within the Regio Magna Occidentalis ("Large Western Region") in his map of c.1600. Pierre Gassendi had included it as part of the 'Homuncio' ('little man'), referring to a small humanoid figure that he could see among the maria; Gassendi also referred to it as 'Thersite' after Thersites, the ugliest warrior in the Trojan War. Michael Van Langren had labelled it the Mare Eugenianum ("Eugenia's Sea") in his 1645 map, in honour of Isabella Clara Eugenia, queen of the Spanish Netherlands. And Johannes Hevelius included it within Pontus Euxinus (after the classical name for the Black Sea) in his 1647 map.

Mare Tranquillitatis is a lunar mare that sits within the Tranquillitatis basin on the Moon. It is the first location on another world to be visited by humans.

The mare material within the basin consists of basalt formed in the intermediate to young age group of the Upper Imbrian epoch. The surrounding mountains are thought to be of the Lower Imbrian epoch, but the actual basin is probably Pre-Nectarian. The basin has irregular margins and lacks a defined multiple-ringed structure. The irregular topography in and near this basin results from the intersection of the Tranquillitatis, Nectaris, Crisium, Fecunditatis, and Serenitatis basins with two throughgoing rings of the Procellarum basin. Palus Somni, on the northeastern rim of the mare, is filled with the basalt that spilled over from Tranquillitatis.

This mare has a slight bluish tint relative to the rest of the Moon and stands out quite well when color is processed and extracted from multiple photographs. The color is likely due to higher metal content in the basaltic soil or rocks.

Unlike many other maria, there is no mass concentration (mascon), or gravitational high, in the center of Mare Tranquillitatis. Mascons were identified in the center of other maria (such as Serenitatis or Imbrium) from Doppler tracking of the five Lunar Orbiter spacecraft in 1968.[4] The gravity field was mapped at higher resolution with later orbiters such as Lunar Prospector and GRAIL, which unveiled an irregular pattern.

Posidonius is a lunar impact crater that is located on the north-eastern edge of Mare Serenitatis, to the south of Lacus Somniorum. It was named after ancient Greek philosopher and geographer Posidonius of Apamea. The crater Chacornac is attached to the southeast rim, and to the north is Daniell.

The rim of Posidonius is shallow and obscured, especially on the western edge, and the interior has been overlain by a lava flow in the past. The crater ramparts can still be observed to the south and east of the crater rim, and to a lesser degree to the north.

There is a smaller, semi-circular rim of a concentric, flooded crater within the main rim, offset towards the eastern edge. There is no central peak, but the floor is hilly and laced with a rille system named the Rimae Posidonius. The floor is also slightly bulged due to the past lava uplift, which also likely produced the complex of rilles. The northeast rim is interrupted by the smaller crater Posidonius B. Within the crater rim, offset just to the west of center is another smaller crater Posidonius A.

On the Mare Serenitatis surface near Posidonius is a notable system of wrinkle ridges that parallel the nearby shore. These are designated the Dorsa Smirnov. At the peak of these ridges is a small craterlet (Posidonius Y) with a diameter of 2 km. This craterlet is surrounded by a patch of high-albedo material, and is an example of a lunar bright spot. This peak was formerly designated Posidonius Gamma (γ).

Posidonius Gamma was first observed by the lunar cartographer Julius Schmidt in 1867, who noted the similarity to the bright patch surrounding the crater Linné.

Le Monnier is the remnant of a lunar impact crater that has been partly inundated by lava flows. It was named after French astronomer Pierre Charles Le Monnier. It is located on the eastern edge of Mare Serenitatis, and the western part of the rim is missing so that it now forms a large bay. To the north is the crater Chacornac.

The interior of this formation is relatively flat and smooth, with no significant craterlets to mark the surface. The outer rim is battered, worn, and notched with past impacts. Only slight wrinkle ridges now mark where the western rim once lay.

The landing site of the Luna 21 space probe is located near the southern rim of Le Monnier. Lunokhod 2, a robotic roving vehicle deposited on the surface by Luna 21, covered a distance of 37 kilometers across the crater floor. It also surveyed the southern edge of the crater rim.