Felimare ruthae is a colourful species of sea slug or dorid nudibranch, a marine gastropod mollusk in the family Chromodorididae.

[5] Felimare ruthae has a blue-black body with a yellow-lined mantle and yellow longitudinal lines and bright-blue striations on its dorsum.

The gills are typically translucent outlined with black and the rhinophores are black with a yellow-white line running vertically.

[6][7] This species can reach a total length of at least 25 mm and has been observed feeding on the sponge Dysidea etheria.

[3] Minimum recorded depth is 1 m (3 ft 3 in) and maximum recorded depth is 12 m (39 ft).