Costasiella is a genus of sacoglossan sea slugs, a shell-less marine opisthobranchid gastropod mollusk in the family Costasiellidae.

[2] The Costasiella is defined by some specific characteristics such as smooth rhinophores which are usually simple with dull tips and flattened at their base.

[3][4] Other features include rounded, tentacular anterior foot corners that exhibit their spatulate shape to enhance mobility and blade-shaped radular teeth with substantially short bases.

[4] Their preferred habitats include areas of soft, muddy sediments around intertidal zones.

[3][4] The most recent diagnosis of the genus Costasiella is by the Dutch malacologist Cornelis Kees Swennen (2007).