Thecacera darwini is a species of sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Polyceridae.

This species was first described from Chile, South America.

It was first collected by Charles Darwin.

This polycerid nudibranch is translucent white in colour, with scattered black spots.

[2] Thecacera darwini feeds on the bryozoan Beania magellanica.