Nembrotha rosannulata is a large dull green nembrothid that grows to at least 12 cm in length. The body is textured with rounded longitudinal ridges and scattered raised pustules of various sizes. The pustules are black and each is surrounded by a pink ring which give this species its characteristiccolourpattern - and name. The rhinophores are black, and the pink rhinophores sheaths are edged in black. The front of the mantle has a pink band which is edged in black, and the oral tentacles are pink with black tips. The gill stalks and branches are pink while the gill pinnae are black.