The Tonian (from Ancient Greek: τόνος, romanized: tónos, meaning "stretch") is the first geologic period of the Neoproterozoic Era. It lasted from 1000 to 720 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined by the ICS based on radiometric chronometry. The Tonian is preceded by the Stenian Period of the Mesoproterozoic Era and followed by the Cryogenian.

The Tonian was defined in 1990 by the New Precambrian time scale as being from 1000 to 850 Mya. The name Tonian comes from Ancient Greek: τόνος, romanized: tónos, which means "stretch", referring to the rifting that broke up Rodinia during this period. The upper definition was then redefined to 720 Mya in 2014 after discoveries that the original 850 Mya for the lower Cryogenian and the first glaciations at 717 Mya did not match. This is temporary until the basal Cryogenian GSSP is chosen.

Both the Supercontinent Rodinia and the Mirovian superocean remained stable throughout the early Tonian from 1000 to at least 900 Mya. The appearance of sedimentary rocks in multiple places in 850 to 800 Mya has been postulated as an initial breakup of Rodinia. However, there has been little evidence for the breakup of Rodinia during the mid-Tonian, with most agreeing that Rodinia began breaking up at the end of the Tonian, around 750 Mya.

The first half of the Tonian has mildly fluctulating carbon isotope values, being transitional from the lower Mesoproterozoic levels to the higher Tonian levels. The later half of the Tonian has high carbon isotope values, with deep excursions happening at least once after 740 Mya, immediately before the Sturtian glaciation in the Cryogenian.

The first putative metazoan (animal) fossils are dated to the middle to late Tonian (c. 890 to 800 Ma). Fossils of Otavia antiqua, which has been described as a primitive sponge by its discoverers and numerous other scholars, date back to about 800 Ma. Even earlier sponge-like fossils have been reported in reefs dating back to 890 Ma, but their identity is highly debated. This dating is consistent with molecular data recovered through genetic studies on modern metazoan species; more recent studies have concluded that the base of the animal phylogenetic tree is in the Tonian.

Tonian rocks preserve some of the earliest known fossils of macroalgae, such as the benthic macroalgae from the Longfengshan biota of the Luotuoling Formation or the green algae from the Dolores Creek Formation. Arctacellularia tetragonala, an alga found in this period, has chlorophyll derivatives preserved in it. Sinosabellidites huainanensis, another alga from the Huainan biota that was initially confused for an animal, was found in this period.

The first large evolutionary radiation of acritarchs occurred during the Tonian. Late Tonian sediments abound in vase-shaped microfossils that represent the earliest known testate amoebozoans. Stromatolites begin to swiftly decrease during this period.

Fungi are visibly preserved in this period in 810 to 715 million year old shale in the Mbuji-Mayi Supergroup with mycelia-like filaments.