An undulipodium or undulopodium (Greek: "swinging foot"; plural undulipodia), or a 9+2 organelle is a motile filamentous extracellular projection of eukaryotic cells. It is basically synonymous to flagella and cilia which are differing terms for similar molecular structures used on different types of cells, and usually correspond to different waveforms.

The name was coined to differentiate from the analogous structures present in prokaryotic cells.

The usage of the term was early supported by Lynn Margulis, especially in support of endosymbiotic theory. The eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length. The Gene Ontology database does not make a distinction between the two, referring to most undulipodia as "motile cilium", and to that in the sperm as sperm flagellum.