The shell of Crepidula onyx is up to 70 mm in length.

The interior of the shell is glossy and is a strong tan to dark brown color, similar to the common colors of the mineral onyx.

The shell of this species can be distinguished from the shell of Crepidula adunca by: The native range for this species is the Eastern Pacific Ocean coast of the Americas, from Southern California to Chile.

The species has been introduced and is considered to be established in Puget Sound, Washington, since 2005.

This species is common on rocks, shells, and pilings in protected bays, but it also lives in sheltered places on the open coast, occurring from the low intertidal zone to subtidal waters.