Nereididae (formerly spelled Nereidae) are a family of polychaete worms. It contains about 500 - mostly-marine - species grouped into 42 genera. They may be commonly called ragworms or clamworms.

The prostomium of Nereididae bears a pair of palps that are differentiated into two units. The proximal unit is much larger than the distal unit. Parapodia are mostly-biramous (only the first two pairs are uniramous). Peristomium fused with the first body-segment, with usually two pairs of tentacular cirri. The first body-segment with 1-2 pairs tentacular cirri without aciculae. Compound setae are present. Notopodia are distinct (rarely reduced), usually with more flattened lobes, notosetae compound falcigers and/or spinigers (rarely notosetae absent). They have two prostomial antennae (absent in Micronereis). Their pharynx, when everted, clearly consists of two portions, with a pair of strong jaws on the distal portion and usually with conical teeth on one or more areas of both portions. Most genera have no gills (if present, they are usually branched and arise on mid-anterior segments of body). The larval body consists of four segments.

Ragworms' teeth are made of a very tough, yet lightweight material. Unlike bone and tooth enamel, this is not mineralized with calcium, but is formed by a histidine-rich protein, with bound zinc ions. Research on this material could lead to applications in engineering.

Nereididae are currently considered a monophyletic taxon. Their closest neighbours in polychaete phylogenetic tree are Chrysopetalidae and Hesionidae (the superfamily Nereidoidea).

Nereididae are divided into 42 genera, but the relationships between them are as yet unclear. The family contains traditionally three subfamilies - Namanereidinae, Gymnonereinae and Nereidinae.

Subfamily Gymnonereidinae Banse, 1977

Subfamily Namanereidinae Hartman, 1959

Subfamily Nereidinae Blainville, 1818