In phylogenetics (a subfield of biology) and in linguistics, a taxon is said to be paraphyletic if the group consists of all the descendants of some ancestor minus one or more monophyletic groups of descendants. For example, the group of reptiles, as traditionally defined, is paraphyletic: it contains the last common ancestor of extant reptiles, mammals, and birds and all descendants of that ancestor, except for mammals and birds.

Groups that do include all the descendants of the most recent common ancestor are said to be monophyletic. A paraphyletic group is a monophyletic group from which one or more of the clades is excluded to form a separate group.

A taxon that is not paraphyletic or monophyletic is polyphyletic (Greek πολύς [polys], "many").

These terms were developed during the debates of the 1960s and 70s accompanying the rise of cladistics (a clade is a term for a monophyletic group).

Many of the older classifications contain paraphyletic groups, especially the traditional 2–6 kingdom systems and the classic division of the vertebrates. Paraphyletic groups are often erected on the basis of (sym)plesiomorphies (ancestral similarities) instead of (syn)apomorphies (derived similarities). Examples of well-known paraphyletic groups include: