Obelia is a genus of hydrozoans, a class of mainly marine and some freshwater animal species that have both polyp and medusa stages in their life cycle. Hydrozoa belongs to the phylum Cnidaria, which are aquatic (mainly marine) organisms that are relatively simple in structure with a diameter around 1mm. There are currently 120 known species, with more to be discovered. These species are grouped into three broad categories: O. bidentata, O. dichotoma, and O. geniculata. O. longissima was later accepted as a legitimate species, but taxonomy regarding the entire genus is debated over.

Obelia is also called sea fur.

Obelia has a worldwide distribution except the high-Arctic and Antarctic seas. and a stage of Obelia species are common in coastal and offshore plankton around the world. Obelia are usually found no deeper than 200 metres (660 ft) from the water's surface, growing in intertidal rock pools and at the extreme low water of spring tides.