Costasiella coronata is a species of sacoglossan sea slug, a shell-less marine opisthobranch gastropod mollusk in the family Costasiellidae.

[2] The type locality for this species is mangrove forest in Bang Tawa, southern Thailand.

[1] The type specimen (holotype) is stored in the Zoological Reference Collection of the Raffles Museum of Biodiversity Research, National University of Singapore.

[1] The renal shield is large and cover 2/3 of the body when the slug is fully extended.

[1] They are found in higher density on Vaucheria when the algae is above the sea water level.