Thysanozoon nigropapillosum is a species of polyclad flatworms belonging to the family Pseudocerotidae.

[2] The dorsal surface is deep black and covered with numerous yellow-tipped papillae varying in size.

[4] Thysanozoon nigropapillosum is quite common along the external reef in the shallow sub-tidal zone.

Transfer of sperm may be by hypodermic insemination, but this is made difficult in this species by the presence of the papillae, and dermal impregnation often occurs.

Such dermal insemination is facilitated by the fact that the flatworm has two penises which are strengthened by the presence of rhabdites, and these can grip the margin of the partner and keep hold, even while the recipient is swimming.