Retidrillia megalacme is a species of sea snail, a marine gastropod mollusk in the family Borsoniidae.

(Original description) The small shell has a conical-fusiform shape.

The remaining whorls are acutely carinate, with an area below the suture, either smooth or with arcuate striae.

Below the carina appear numerous longitudinal riblets, decussated by spiral carinations, giving the shell a somewhat prickly or nodulous appearance.

[2] This species occurs in the Atlantic Ocean off Portugal.