In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—monophyletic subgroups. The group is said to be paraphyletic with respect to the excluded subgroups. A paraphyletic group cannot be a clade, or monophyletic group, which is any group of species that includes a common ancestor and all of its descendants. One or more members of a paraphyletic group is more closely related to the excluded group(s) than it is to other members of the paraphyletic group. The term is commonly used in phylogenetics (a subfield of biology) and in linguistics. Paraphyletic groups are identified by a combination of synapomorphies and symplesiomorphies.

In this phylogenetic tree, the green group is paraphyletic; it is composed of a common ancestor (the lowest green vertical stem) and its descendants, but it excludes the blue group (a monophyletic group) which diverged from the green group.

The term was coined by Willi Hennig to apply to well-known taxa like Reptilia (reptiles) which, as commonly named and traditionally defined, is paraphyletic with respect to mammals and birds. Reptilia contains the last common ancestor of reptiles and all descendants of that ancestor, including all extant reptiles as well as the extinct synapsids, except for mammals and birds. Other commonly recognized paraphyletic groups include fish, monkeys, and lizards.[1][page needed]

If many subgroups are missing from the named group, it is said to be polyparaphyletic.