Indole-3-carbaldehyde (I3A), also known as indole-3-aldehyde and 3-formylindole, is a metabolite of dietary L-tryptophan which is synthesized by human gastrointestinal bacteria, particularly species of the Lactobacillus genus.

[2][3] I3A is a biologically active metabolite which acts as a receptor agonist at the aryl hydrocarbon receptor in intestinal immune cells, in turn stimulating the production of interleukin-22 which facilitates mucosal reactivity.

[4][3][2] Indole-3-carbaldehyde has reactivity typical of aromatic aldehydes.

It condenses with nitromethane in a Henry reaction to give 3-nitrovinyl indole.

Indole-3-carbaldehyde has antifungal properties, and partially accounts for the protection from chytridiomycosis seen in amphibian species which carry Janthinobacterium lividum on their skin.