Acetoacetic acid (also known as 3-oxobutanoic acid or diacetic acid) is a beta-keto acid of the keto acid group, its empirical formula is C₄H₆O₃ or CH₃COCH₂COOH.

It is a strong organic acid and can be produced in the human liver under certain conditions of poor metabolism leading to excessive fatty acid breakdown (diabetes mellitus leading to diabetic ketoacidosis), it is then partially converted to acetone by decarboxylation and excreted either in urine or through respiration. The acid is also present in the metabolism of those undergoing starvation or prolonged physical exertion as part of gluconeogenesis. It is not the major ketone produced by the body (that being beta-hydroxybutyrate ).

When ketone bodies are measured by way of urine concentration, acetoacetic acid, along with beta-hydroxybutyric acid or acetone, is what is detected. This is done using dipsticks coated in nitroprusside or similar chemicals. Nitroprusside changes from pink to purple in the presence of acetoacetate, the conjugate base of acetoacetic acid, and the colour change is graded by eye.