Mare Tranquillitatis ("sea of tranquility") is a lunar mare that sits within the Tranquillitatis basin on Earth's Moon. The mare material within the basin consists of basalt 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.

In 1965, the Ranger 8 spacecraft crashed in Mare Tranquillitatis, after successfully transmitting 7,137 photographs of the moon in the final 23 minutes of its mission. This mare also served as the landing site for Apollo 11, the first manned landing on the Moon.