脂质A（英语：lipid A）为一种糖磷脂，由D-氨基葡萄糖双糖组成的基本骨架，不同细菌骨架一致。具有内毒素毒性与生物学活性成分，无种属特异性。

Lipid A is a lipid component of an endotoxin held responsible for the toxicity of gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide (LPS), also called endotoxin molecule, and its hydrophobic nature allows it to anchor the LPS to the outer membrane. While its toxic effects can be damaging, the sensing of lipid A by the human immune system may also be critical for the onset of immune responses to gram-negative infection, and for the subsequent successful fight against the infection.