In cladistics, a synapomorphy or synapomorphic character is a trait that is shared ("symmorphy") by two or more taxa and their most recent common ancestor, whose own ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in their last common ancestor. The word "synapomorphy" is derived from the Greek words σύν, syn = with, in company with, together with; ἀπό, apo = away from; and μορφή, morphe = shape.

True synapomorphies usually uniquely characterise a given set of terminal groups, but this is not essential to the concept. Thus, if some descendants of a last common ancestor possess a synapomorphic trait, it is not strictly necessary that all of its descendants must possess the same trait.

A synapomorphy should not be confused with other types of shared traits:

Synapomorphies are used to establish phylogenies in cladistic analyses. As such they are empirical data which can support a certain hypothesis that terminal groups form a clade (monophyletic group) together to the exclusion of certain other groups – whereas character-states that are shared, but also shared by other terminal groups descending from an earlier common ancestor, cannot be used to exclude these other groups. The latter character-states can consist of symplesiomorphies ("primitive" character-states having originated in the earlier common ancestor) or homoplasies (superficially similar but independently evolved derived character-states).