A cladistic analysis and taxonomic revision of the Namanereidinae (Nereididae: Polychaeta) is presented. The cladistic analysis utilising 39 morphological characters (76 apomorphic states) yielded 10,000 minimal-length trees and a highly unresolved Strict Consensus tree. However, monophyly of the Namanereidinae is supported and two clades are identified: Namalycastis containing 18 species and Namanereis containing 15 species. The monospecific genus Lycastoides, represented by L. alticola Johnson, is too poorly known to be included in the analysis. Classification of the subfamily is modified to reflect the phylogeny. Thus, Namalycastis includes large-bodied species having four pairs of tentacular cirri; autapomorphies include the presence of short, subconical antennae and enlarged, flattened and leaf-like posterior cirrophores. Namanereis includes smaller-bodied species having three or four pairs of tentacular cirri; autapomorphies include the absence of dorsal cirrophores, absence of notosetae and a tripartite pygidium. Cryptonereis Gibbs, Lycastella Feuerborn, Lycastilla Solís-Weiss & Espinasa and Lycastopsis Augener become junior synonyms of Namanereis. Thirty-six species are described, including seven new species of Namalycastis (N. arista n.sp., N. borealis n.sp., N. elobeyensis n.sp., N. intermedia n.sp., N. macroplatis n.sp., N. multiseta n.sp., N. nicoleae n.sp.), four new species of Namanereis (N. minuta n.sp., N. serratis n.sp., N. stocki n.sp., N. sublittoralis n.sp.), and three widespread species groups (Namalycastis abiuma, Namanereis littoralis, N. quadraticeps). Fourteen species are newly placed into synonymy, Lycastis maxillo-falciformis Harms, L. maxillo-ovata Harms, L. maxillo-robusta Harms, Lycastis meraukensis Horst, L. nipae Pflugfelder, L. ouanaryensis Gravier, L. ranauensis Feuerborn, L. vivax Pflugfelder, Lycastopsis augeneri Okuda, L. tecolutlensis Rioja, Namalycastis rigida Pillai, N. tachinensis Rosenfeldt, N. vuwaensis Ryan, and Namanereis littoralis Hutchings & Turvey. A neotype is designated for Namalycastis hawaiiensis (Johnson), and lectotypes are designated for Namalycastis geayi (Gravier), N. senegalensis (Saint-Joseph), N. terrestris (Pflugfelder), Namanereis amboinensis (Pflugfelder) and N. littoralis (Grube). Keys to genera and species are given. Namanereidinae are generally confined to the tropics and subtropics. Maximum species-diversity occurs in the Caribbean and Indo-Pacific, in particular in coastal areas subjected to recent uplifting, where both littoral-zone and freshwater (riparian and subterranean) forms occur. Phylogenetic results indicate that in both Namalycastis and Namanereis there is a preference for freshwater habitats among species with apomorphic traits (corollary being that marine habitats are favoured by the plesiomorphic members). This suggests that the ancestor of the Namanereidinae was a euryhaline coastal species.