Nodulation (nod) factors are signal molecules produced by bacteria known as rhizobia during the initiation of nodules on the root of legumes.

Nod factors structurally are lipochitooligosaccharides that consist of an acylated chitin oligomeric backbone with various functional group substitutions at the terminal or non-terminal residues. The exact chemical structure of the nod factor that is recognised by the plant varies between species and is the basis for host-symbiont specificity.