Ursactis is an extinct genus of Cambrian polychaete annelid.

[1] It was found in the Wuliuan Stage of the Burgess Shale, although most specimens come from a Burgess Shale locality known as the Tokumm Creek in northern Kootenay National Park.

[1] Ursactis is the most abundant Cambrian polychaete found as of 2023.

[1] Ursactis is small species, 3–15 millimetres (0.1–0.6 in) long, with a pair of large palps.

This suggests that annelids began to start adding new segments during their life by the mid-Cambrian.