Histone deacetylase (HDAC) inhibitors represent an emerging class of anticancer agents progressing through clinical trials. Although their primary target is thought to involve acetylation of core histones, several nonhistone substrates have been identified, including heat shock protein (HSP) 90, which may contribute towards their antitumor activity. Glucose-regulated protein 78 (GRP78) is a member of the HSP family of molecular chaperones and plays a central role in regulating the unfolded protein response (UPR). Emerging data suggest that GRP78 is critical in cellular adaptation and survival associated with oncogenesis and may serve as a cancer-specific therapeutic target. On the basis of shared homology with HSP family proteins, we sought to determine whether GRP78 could serve as a molecular target of the HDAC inhibitor vorinostat. Vorinostat treatment led to GRP78 acetylation, dissociation, and subsequent activation of its client protein double-stranded RNA-activated protein-like endoplasmic reticulum kinase (PERK). Investigations in a panel of cancer cell lines identified that UPR activation after vorinostat exposure is specific to certain lines. Mass spectrometry performed on immunoprecipitated GRP78 identified lysine-585 as a specific vorinostat-induced acetylation site of GRP78. Downstream activation of the UPR was confirmed, including eukaryotic initiating factor 2alpha phosphorylation and increase in ATF4 and C/EBP homologous protein expression. To determine the biologic relevance of UPR activation after vorinostat, RNA interference of PERK was performed, demonstrating significantly decreased sensitivity to vorinostat-induced cytotoxicity. Collectively, these findings indicate that GRP78 is a biologic target of vorinostat, and activation of the UPR through PERK phosphorylation contributes toward its antitumor activity.