Demosponges or common sponges are sponges of the class Demospongiae (from Ancient Greek: δῆμος, romanized: dêmos, lit. 'common people' + σπογγιά, spongiá, 'sponge'), the most diverse group in the phylum Porifera which include greater than 90% of all extant sponges with nearly 8,800 species worldwide (according to the World Porifera Database). Being siliceous sponges, they are predominantly leuconoid in structure with an endoskeleton made of a meshwork of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges. Some species, in particular from the Antarctic, obtain the silica for spicule-building from the ingestion of diatoms.

The many diverse orders in this class include all of the large sponges. About 311 million years ago, in the Late Carboniferous, the order Spongillida split from the marine sponges, and is the only sponges to live in freshwater environments. Some species are brightly colored, with great variety in body shape; the largest species are over 1 m (3.3 ft) across. They reproduce both sexually and asexually. They are the only extant organisms that methylate sterols at the 26-position, a fact used to identify the presence of demosponges before their first known unambiguous fossils.

Because of many species' long life span (500–1,000 years) it is thought that analysis of the aragonite skeletons of these sponges could extend data regarding ocean temperature, salinity, and other variables farther into the past than has been previously possible. Their dense skeletons are deposited in an organized chronological manner, in concentric layers or bands. The layered skeletons look similar to reef corals. Therefore, demosponges are also called coralline sponges.

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