Thecacera velox Cockerell, 1901 original combination Trapania velox is a species of sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Goniodorididae.

[2] This species was described from La Jolla, California.

[3] This goniodorid nudibranch is translucent white in colour, with a pattern dark brown lines forming a loose network on the body.

The tips of the rhinophores, oral tentacles, gills, tail and lateral papillae are tipped with yellow.

[4] Trapania velox probably feeds on Entoprocta which often grow on sponges and other living substrata.