Longchaeus mazatlanicus is a species of sea snail, a marine gastropod mollusk in the family Pyramidellidae, the pyrams and their allies.

It has a horn color, with a little darker band on the middle between the sutures, which is bordered at its anterior margin by a faint light line.

They are scarcely at all contracted at the suture, with a narrow, very finely crenulated shoulder at the summit.

The periphery of the body whorl is strongly, sharply, and deeply sulcate.

The base of the shell is short, well rounded, and with a strong fasciole at the insertion of the columella.