Agathotoma castellata is a species of sea snail, a marine gastropod mollusk in the family Mangeliidae.

(Original description) The white, oblong, turreted shell has 6 to 7 whorls, including two in the protoconch.

This shell is elegantly formed, with whorls that are narrower at the base than above, giving it a turreted appearance.

The ribs are slightly extended at the upper end, giving the whorls a castellated appearance, and they are continuous up the spire in all four examples examined.

[2] This species occurs in the Caribbean Sea off Guadeloupe, Aruba; Curaçao and Venezuela; also off the Bahamas at depths between 3 and 21 m.; also off Brazil.