Histone deacetylase 4 (HDAC4) is a class II HDAC implicated in controlling gene expression important for diverse cellular functions, but little is known about how its expression and stability are regulated. We report here that this deacetylase is unusually unstable, with a half-life of less than 8 h. Consistent with the instability of HDAC4 protein, its mRNA was also highly unstable (with a half-life of less than 4 h). The degradation of HDAC4 could be accelerated by exposure of cells to ultraviolet irradiation. HDAC4 degradation was not dependent on proteasome or CRM1-mediated export activity but instead was caspase-dependent and was detectable in diverse human cancer lines. Of two potential caspase consensus motifs in HDAC4, both lying within a region containing proline-, glutamic acid-, serine-, and threonine-rich (PEST) sequences, we identified, by site-directed mutagenesis, Asp-289 as the prime cleavage site. Notably, this residue is not conserved among other class IIa members, HDAC5, -7, and -9. Finally, the induced expression of caspase-cleavable HDAC4 led to markedly increased apoptosis. These results therefore unexpectedly link the regulation of HDAC4 protein stability to caspases, enzymes that are important for controlling cell death and differentiation.