The biuret test is a chemical test used for detecting the presence of peptide bonds. In the presence of peptides, a copper(II) ion forms a violet-colored complex in an alkaline solution. Several variants on the test have been developed.

The Biuret reaction can be used to assay the concentration of proteins because (for most proteins) peptide bonds occur with approximately the same frequency per gram of material. The intensity of the color, and hence the absorption at 540 nm, is directly proportional to the protein concentration, according to the Beer-Lambert law.

In spite of its name, the reagent does not in fact contain biuret ((H₂N-CO-)₂NH). The test is so named because it also gives a positive reaction to the peptide bonds in the biuret molecule.