Caspases play a central and evolutionarily conserved role in mediating and executing apoptosis. Here, we report the cloning and characterization of a caspase from Penaeus monodon, Pm caspase. The full-length Pm caspase cDNA is 1386bp, encoding a polypeptide of 304 amino acids with a calculated molecular mass of 34.3kDa. BLASTP analysis against the NCBI nr database showed that Pm caspase is similar to insect effector caspases. RT-PCR analysis showed that Pm caspase mRNA is expressed in all examined tissues. When Pm caspase was overexpressed in SF-9 cells, the cells showed apoptotic morphological features, including the formation of apoptotic bodies and DNA ladders. The caspase-3 activity of Pm caspase was determined using the recombinant protein purified from Escherichia coli. Both RT-PCR and qRT-PCR analyses showed that the RNA levels of Pm caspase and P. monodon inhibitor of apoptosis protein (PmIAP) remained unchanged after white spot syndrome virus (WSSV) infection. We also used Pm caspase to show that WSSV449, an anti-apoptosis protein encoded by WSSV, is a direct caspase inhibitor.