Oceanus Procellarum, Latin for "Ocean of Storms", is a vast lunar mare on the western edge of the near side of Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather. Procellarum is the largest of the lunar maria, stretching 2500 kilometers across its north-south axis and covering roughly 4,000,000 km².

Like all lunar maria, Oceanus Procellarum was formed by ancient basaltic flood volcanic eruptions that covered the region in a thick, nearly flat layer of solidified magma. Unlike the other lunar maria, however, Procellarum is not contained within a single well-defined impact basin (although much of it is within the Procellarum Basin ). Around its edges lie many minor bays and seas, including Mare Nubium and Mare Humorum to the south. To the northeast, Oceanus Procellarum is separated from Mare Imbrium by the Carpathian Mountains.

The unmanned lunar probes Surveyor 1, Surveyor 3, Luna 9 and Luna 13 landed in Oceanus Procellarum. Apollo 12 also landed in Oceanus Procellarum.