Onchidoris bilamellata, common name the rough-mantled doris, is a species of sea slug, a dorid nudibranch, a shell-less marine gastropod mollusk in the family Onchidorididae.

These sea slugs sometimes occur in abundant numbers during breeding time, swarming with over 1,000 individuals per square metre, as has been observed on numerous occasions.

Onchidoris bilamellata has a broadly oval shape with its back covered with short, knobby papillae.

The gills are arranged in two half rings on the back at the rear end and consist of 16 to 32 simple featherlike plumes.

It uses its rachidian tooth to chew away at the top plates of the barnacles and then sucks out the soft body.