The Stenian Period (/ˈstiːni.ən/ STEE-nee-ən, from Ancient Greek: στενός, romanized: stenós, meaning "narrow") is the final geologic period in the Mesoproterozoic Era and lasted from 1200 Mya to 1000 Mya (million years ago).

The name derives from narrow polymetamorphic belts formed over this period.

It would last into the Tonian period before breaking up in the Cryogenian.

This period includes the formation of the Keweenawan Rift at about 1100 Mya.

[5] Fossils of the oldest known sexually reproducing organism, Bangiomorpha pubescens, first appeared in the Stenian.