Microsomal triglyceride transfer protein (MTP) is needed to assemble chylomicrons in the endoplasmic reticulum (ER) of enterocytes. We explored the role of an ER stress protein, inositol-requiring enzyme 1β (IRE1β), in regulating this process. High-cholesterol and high-fat diets decreased intestinal IRE1β mRNA in wild-type mice. Ire1b^−/− mice fed high-cholesterol and high-fat diets developed more pronounced hyperlipidemia because these mice secreted more chylomicrons and expressed more intestinal MTP, though not hepatic MTP, than wild-type mice did. Chylomicron secretion and MTP expression also were increased in primary enterocytes isolated from cholesterol-fed Ire1b^−/− mice. There was no correlation between ER stress and MTP expression. Instead, cell culture studies revealed that IRE1β, but not its ubiquitous homolog IRE1α, decreased MTP mRNA through increased posttranscriptional degradation. Conversely, knockdown of IRE1β enhanced MTP expression. These studies show that IRE1β plays a role in regulating MTP and in chylomicron production.