Shell of about 2.1-2.5 mm in length, ovoid, stout, with a low spire. White-yellowish, translucent, with three spiral bands of light brown colour badly defined but constant. The upper band appears near the suture. The two others are on the middle of the body whorl, are the most evident and reach the end of the spire (the central one ends a little before). On the base, close to the siphonal canal, there is a fourth band, also light brown. Aperture elongate, outer lip thickened, denticulate, with about 17 denticles on the inner side, which are well defined, extended along the entire aperture, a little separated from the lip border. Columella with 4 (sometimes 5) plaits, decreasing in size posteriorly. Siphonal canal distinctly notched.
Animal with the foot translucent white, with milky white blotches regularly disposed near the border along it, mixed with a variable number or orange spots. Head bearing two cylindrical translucent tentacles and two conspicuous anterior lobules. Several orange spots are set on a line which runs from behind the eyes, along their inner side and it finish at the base of the tentacles. The siphon is translucent white. The internal part of the mantle is clearly seen through the shell and shows three very irregular blotches: one in the spire and two in the body whorl. These blotches are bordered of black and have orange and frequently elongated spots near the black border. The rest have predominantly smaller light green spots bordered of black. The space between the blotches is not coloured. When the animal is placed in alcohol the coloration of the soft parts is almost lost, holding only a yellowish tone with some violet secretion.
On rocks with small mats of algae and sand, from the intertidal zone to 6 metres deep.

The sympatric G. quadrifasciata is bigger, more solid and has a different animal colour. The more similar species to G. decorfasciata, is G. diadema Pin & Boyer 1995, that differs because the shell of this last species is translucent without any brown band, and is a little bigger and relatively slender; a constant difference is that the denticles are along the whole external lip in G. decorfasciata while practically they disappear in the upper half of G. diadema. The profile ot G. diadema is regularly ovoid while in G. decorfasciata the spire and the external lip of the aperture are more prominent. There are also differences in the colour ol soft parts: G. diadema has head and foot of translucent light grey colour with big white dots and others black: G. decorfasciata has the foot white translucent with regular and symmetrical radial white blotches, and never black dots. The tentacles of G. diadema have a prominent white spot which does not exist in G. decorfasciata. Finally, the central part visible by transparency in the dorsum of the shell is light yellow or greenish yellow in G. diadema, while it is totally white, sometimes with orange spots in G. decorfasciata. Also is very typical in G. decorfasciata an orange line in each side of the head which is substituted by orange spots in G. diadema.

Most of the Gibberula species present in the Mediterranean Sea or in the North of Africa (see Gofas, 1990) are bigger and have a very different animal. The exception is G. philippii (Monterosato, 1878) which is small and has a very variable animal, with some similarity to both species. Anyway, G. decorfasciata has brown bands on the shell, the animal colour is constant with dark blotches visible by transparency and no dark colour on the foot (usual in G. philippii).
G. jansseni Aartsen, Menkhorst and Gittenberger, 1984 is also of a similar size but it has the animal not so dark, with more orange dots visible by transparency, and the shell is more elongate.
G. rauli Fernandes, 1987 from Cape Verde Islands have a very different colour of soft parts lacking of any spiral band on the shell.