C/EBP homologous protein (CHOP), a transcription factor for the expression of apoptosis-related genes, plays an important role in endoplasmic reticulum (ER) stress-related organ diseases, including diseases of the kidney. Here, we investigated the role of CHOP in ischemia/reperfusion (I/R)-induced acute kidney injury using CHOP-knockout (CHOP^−/−) and wild type (CHOP^+/+) mice. Fifteen or thirty minutes of bilateral renal ischemia (I/R) insult resulted in necrotic and apoptotic tubular epithelial cell death, together with increases in plasma creatinine (PCr) and blood urea nitrogen (BUN) concentrations. After I/R, BiP/GRP78 and CHOP expressions in the kidney gradually increased over time. CHOP expression was greater in the outer medulla than that in the cortex and localized intensely in the nucleus. I/R caused apoptosis of tubular epithelial cells in both CHOP^−/− and CHOP^+/+ mice. The number of apoptotic cells after I/R was lower in CHOP^−/− mice than that in CHOP^+/+ mice. Consistent with the degree of apoptosis, I/R-induced kidney morphological and functional damages were milder in CHOP^−/− than that in CHOP^+/+ mice. The cleavage of procaspase-3 and the induction of Bax protein after I/R were lower in CHOP^−/− than that in CHOP^+/+ mice. In contrast, the expression levels of Bcl-2, Bcl-xL, cIAP2, Mcl-1, and XIAP were higher in CHOP^−/− than that in CHOP^+/+ mice. These results indicate that I/R induces ER stress, leading to the activation of CHOP-associated apoptosis signals, resulting in renal functional and histological damages.