The Rhyacian () is the second geologic period in the Paleoproterozoic Era. It spans 250 million years and lasted from to million years ago (Ma), following the Siderian Period and preceding the Orosirian Period. These dates are defined chronometrically rather than stratigraphically.
The name Rhyacian is derived from the Greek word rhyas, meaning "stream of lava", and refers to the layered intrusions of the Bushfeld Complex in South Africa. The term was proposed by the Subcommission on Precambrian Stratigraphy as a subdivision of the Proterozoic Eon, and was ratified by the International Union of Geological Sciences in 1990. In 2012, there were suggestions to replace the Rhyacian with an alternate name, bearing a time interval of 2250 to 2060 Ma, and decided on the basis of its stratigraphy. The name Jatulian was proposed in reference to the Lomagundi-Jatuli excursion event, while the term Eukaryian was chosen due to the period's existing signs of the earliest eukaryotic fossils. , a replacement for the Rhyacian has not been officially adopted by the IUGS. The term Jatulian, however, has been used in the regional stratigraphy of Fennoscandia.
The Bushveld Igneous Complex and some other similar intrusions formed during this period.
The Huronian (Makganyene) global glaciation began at the start of the Siderian and lasted ~300 million years, continuing through around 40% of the Rhyacian.
Eukaryotes are thought to have originated during the Rhyacian, after an endosymbiotic event between asgardarchaea and alphaproteobacteria; sexual reproduction, a strategy unique to eukaryotes, is also thought to have developed during this period. Possible signs of macroscopic life have been found in Rhyacian aged rocks, although these are heavily disputed due their age and probable abiotic origins.