Oceanus Procellarum (/oʊˈsiːənəs ˌprɒsɛˈlɛərəm/ oh-SEE-ə-nəs PROSS-el-AIR-əm; from Latin: Ōceanus procellārum, lit. 'Ocean of Storms') is a vast lunar plain on the western edge of the near side of the Moon. It is the only one of the lunar plains to be called an "Oceanus" (ocean), due to its size: Oceanus Procellarum is the largest lunar plain, stretching more than 2,500 km (1,600 mi) across its north–south axis and covering roughly 4,000,000 km² (1,500,000 sq mi), accounting for 10.5% of the total lunar surface area.

Like lunar maria, Oceanus Procellarum was formed by ancient volcanic eruptions resulting in basaltic floods that covered the region in a thick, nearly flat layer of solidified magma. Basalts in Oceanus Procellarum have been estimated to be as young as one billion years old. Unlike the lunar maria, however, Oceanus Procellarum may or may not be contained within a single, well-defined impact basin.

Around its edges lie many minor bays and seas, including Sinus Roris to the north, and Mare Nubium, Mare Humorum and Sinus Viscositatis [it] to the south. To the northeast, Oceanus Procellarum is separated from Mare Imbrium by the Carpathian Mountains. On its north-west edge lies the 32 km wide Aristarchus ray crater, the brightest feature on the Near side of the Moon. Also, the more-prominent ray-crater Copernicus lies within the eastern edge of the mare, distinct with its bright ray materials sprawling over the darker material.

There are several hypotheses about the origin of Oceanus Procellarum and a related asymmetry between the near and far sides of the Moon. One of the most likely is that Procellarum was a result of an ancient giant impact on the near side of the Moon. The size of the impact basin has been estimated to be more than 3,000 kilometers, which would make it one of the three largest craters in the Solar System.

The impact likely happened very early in the Moon's history: at the time when magma ocean still existed or had just ceased to exist. It deposited 5–30 km of crustal material on the far side forming highlands. If this is the case, all impact related structures such as crater rim, central peak etc. have been obliterated by later impacts and volcanism. One piece of evidence in support of this hypothesis is concentration of incompatible elements (KREEP) and low calcium pyroxene around Oceanus Procellarum.

Procellarum may have also been formed by spatially inhomogeneous heating during the Moon's formation. The GRAIL mission, which mapped the gravity gradients of the Moon, found square formations resembling rift valleys surrounding the region beneath the lava plains, suggesting the basin was formed by heating and cooling of the lunar surface by internal processes rather than by an impact, which would have left a round crater.

Other hypotheses include a late accretion of a companion Moon on the far side. The latter postulates that in addition to the present Moon, another smaller (about 1,200 km in diameter) moon was formed from debris of the giant impact. After a few tens of millions of years it collided with the Moon and due to a small collisional velocity simply piled up on one side of the Moon forming what is now known as far side highlands.

Relatively recent (less than 2 bya) volcanic activity had been suspected in the Oceanus Procellarum due to the presence of relatively uneroded features. The 2020 Chang'e-5 sample return mission provided constraints on the age of Oceanus Procellarum, finding it to be 1963 ± 57 million years old – over a billion years younger than any other previously returned lunar sample. Late lunar volcanic activity was considered surprising as the Moon is much smaller than Earth; interior heat necessary for volcanism should have been lost three billion years ago, so volcanic rocks as late as those found in Oceanus Procellarum must require additional heat sources.