Endotoxin-bacterial lipopolysaccharide (LPS)-is a driver of lethal infection sepsis through excessive activation of innate immune responses. When delivered to the cytosol of macrophages, cytosolic LPS (cLPS) induces the assembly of an inflammasome that contains caspases-4/5 in humans or caspase-11 in mice. Whereas activation of all other inflammasomes is triggered by sensing of pathogen products by a specific host cytosolic pattern recognition receptor protein, whether pattern recognition receptors for cLPS exist has remained unclear, because caspase-4, caspase-5, and caspase-11 bind and activate LPS directly in vitro. Here, we show that the primate-specific protein NLRP11 is a pattern recognition receptor for cLPS that is required for efficient activation of the caspase-4 inflammasome in human macrophages. In human macrophages, NLRP11 is required for efficient activation of caspase-4 during infection with intracellular Gram-negative bacteria or upon electroporation of LPS. NLRP11 could bind LPS and separately caspase-4, forming a high-molecular weight complex with caspase-4 in HEK293T cells. NLRP11 is present in humans and other primates but absent in mice, likely explaining why it has been overlooked in screens looking for innate immune signaling molecules, most of which have been carried out in mice. Our results demonstrate that NLRP11 is a component of the caspase-4 inflammasome activation pathway in human macrophages.