The Tonian (from Ancient Greek: τόνος, romanized: tónos, meaning "stretch") is the first geologic period of the Neoproterozoic Era.

Rifting leading to the breakup of supercontinent Rodinia, which had formed in the mid-Stenian, occurred during this period, starting from 900 to 850 Mya.

The fossils of Otavia antiqua, which has been described as a primitive sponge by its discoverers and numerous other scholars, date back to about 800 mya.

Even earlier sponge-like fossils have been reported in reefs dating back to 890 million years before the present, but their identity is highly debated.

Vase-shaped microfossils abound in late Tonian sediments and represent the earliest testate amoebozoans.