Alloisoleucine is an amino acid with the formula . It is the diastereomer of isoleucine, differing in the stereochemistry within the side chain. Alloisoleucine exists as two enantiomers, of which the <small>L</small> isomer occurs naturally. <small>L</small>-Alloisoleucine occurs in healthy serum in only trace amounts, except for individuals suffering from maple syrup urine disease.
Together with valine, leucine, and isoleucine, alloisoleucine is classified as a branched-chain amino acid (BCAA). The <small>L</small> enantiomer of valine, leucine, and isoleucine are proteinogenic amino acids; alloisoleucine is non-proteinogenic.
<small>L</small>-Alloisoleucine is a precursor to coronatine, a phytotoxin produced by Pseudomonas syringae.