Caspase 9 (CASP9) is a member of the cysteine-aspartic acid protease (caspase) family that is involved in the execution-phase of cell apoptosis. Caspase 9 is expressed as an inactive proenzyme which then undergos proteolytic processing by upstream caspases, to form an active (cleaved) caspase 9 form.

There are two classes of caspases involved in apoptosis; initiators (activation by receptor cluster) and effectors (activation by mitochondrial permeability transition). Caspase 3 is considered an initiator, as initiated caspase-9 goes on to cleave the effector procaspase-3 and procaspase-7.

Caspase 9 antibodies are important tools for apoptosis studies and research on the caspase family mediated apoptosis signaling pathway.