An abzyme (from antibody and enzyme), also called catmab (from catalytic monoclonal antibody), is a monoclonal antibody with catalytic activity. Molecules which are modified to gain new catalytic activity are called synzymes. Abzymes are usually artificial constructs, but are also found in normal humans (anti-vasoactive intestinal peptide autoantibodies) and in patients with autoimmune diseases such as systemic lupus erythematosus, where they can bind to and hydrolyze DNA. Abzymes are potential tools in biotechnology, e.g., to perform specific actions on DNA.

Enzymes function by lowering the activation energy of the transition state, thereby catalyzing the formation of an otherwise less-favorable molecular intermediate between reactants and products. If an antibody is developed to a stable molecule that's similar to an unstable intermediate of another (potentially unrelated) reaction, the developed antibody will enzymatically bind to and stabilize the intermediate state, thus catalyzing the reaction. A new and unique type of enzyme is produced.