Thuridilla gracilis is a species of sea slug, a sacoglossan, a marine gastropod mollusk in the family Plakobranchidae.

It is an Indo-Pacific species that lives in or near coral reefs and eats algae.

The body is partially covered by the fold of the lateral parapodia.

The mantle is black and crossed by multiple fine white longitudinal lines (eight to ten) punctuated sometimes with blue spots.

Because of its aposematic coloration, it crawls around to feed in the daylight.