Poseidonis is a collection of fantasy short stories by Clark Ashton Smith, edited by Lin Carter.

It was the fourth collection of Smith's works assembled by Carter for the series.

The stories were originally published in various fantasy magazines in the 1930s and 1940s, notably Weird Tales.

[1] The book collects several prose poems, poems and tales, including stories from the author's Poseidonis cycle, set on a remnant of the lost continent of Atlantis, and others set in Lemuria or other realms.

The collection was reviewed (in French) by Daniel Walther in Fiction no, 322, 1981.