(The following description is mainly after a 50 cm diameter specimen described by Russell and Rees, 1960). A deep-sea species, sometimes upwelled. Diameter recorded up to 140 cm, most specimens 50-100 cm. A large, flat, deep brown to maroon medusa; lacking marginal tentacles; with very long, narrow mouth-arms; with annular exumbrellar groove around central half of disc, approximately outlining stomach; and, peripheral to this, prominent anastomosing gastrovascular canals alternate ones of which in many, but not all, specimens form a back-to-back-B pattern. Disc wide, raised in centre; in life, sloping out to perimeter with annular groove halfway to margin; when out of water, flattening under own weight; surface smooth. Jelly thick and firm, thickest beyond margin of stomach (i.e. immediately peripheral to annular groove), tapering peripherally. Marginal lappets shallow, rounded, 40 rhopalial and 20 inter-rhopalial. Rhopalia 20, simple, situated at ends of "B-pattern" radial canals. Subumbrellar musculature annular, weakly developed, in area of thickened jelly. Mouth cross-shaped, small. Mouth-arms 3 or more times as long as disc diameter, united basally to form a tube round in section and somewhat massive; distal ca. 3/4 comprising two thin, flat flanges with groove between; the margins finely scalloped, diameter of crenations ca. 5 mm. Stomach large, much modified to enclose four quadrant-shaped brood chambers each having thick-bordered ca. 15 mm diameter aperture through stomach floor. Numerous gastric cirri, ca. 5 mm long, armed with nematocysts, surrounding each brood pouch. Radial canals 40, conspicuous, straight to variously tortuous; gastrovascular network in two zones; alternate radial canals crossing inner zone unbranched (inter-rhopalial canals) and without lateral connections, those between (rhopalial canals) having broad, looping side-branches typically in back-to-back-B pattern (this irregular and not discernible in some specimens). In outermost zone all 40 radial canals distinct, and all similarly connected by anastomosis of irregular mesh about 1/2 as large as inner "B-pattern" zone. Ring canal following the divided margin. Gonads 4, in stomach pouches, gametes perhaps developing in germinal line on outer surface of brood pouches. Embryo brooded in unique cyst, probably a modified scyphistoma, with umbilical connection to gastrovascular system; young released viviparously at large size.
Coloration: generally "a deep brown-red or plum color typical of deep-sea animals", pigment cells present right through jelly (Russell and Rees, 1960); radial canals and anastomoses whitish, contrasting and conspicuous.
Remark: Stygiomedusa fabulosa Russell, 1959, and Stygiomedusa stauchi Repelin, 1967, are conspecific (Cornelius, 1973; Larson, 1986).