Adult female Rhynchactis reach a standard length (SL) of 11–13 cm (4.3–5.1 in) and have a dark-colored, streamlined body and a relatively small head bearing a very long illicium (the "fishing rod" formed by the first ray of the dorsal fin).

The lack of an esca, greatly reduced dentition, and glands inside the mouth all point to Rhynchactis having a highly specialized mode of feeding, the nature of which has yet to be deciphered.

Uniquely, the inside of both jaws are densely covered by white, papilla-like glands, each containing a short, pigmented tube at the center lined by glandular cells.

As in other deep-sea anglerfishes, the adult male is much smaller than the female and has no illicium; the eyes are minute while the olfactory organs are highly developed, measuring 8–10% of the standard length.

[2][3] The species differ from each other as follows: Without a luminous esca to attract prey or large teeth to secure it, Rhynchactis seems to have evolved a completely novel, and as yet unknown, mode of feeding.

The larvae have short, stout bodies and highly inflated skin that give them an almost spherical shape; their pectoral fins are very large, measuring about half as long as the standard length.