Caspase-4 and caspase-5 are human-specific inflammatory proteases that detect cytoplasmic lipopolysaccharide (LPS), and are thus comparable to the now well-described, murine-specific caspase-11. When activated, these trigger pyroptotic cell death and caspase-1-dependent IL-1β production. We have shown that a specific NLRP3 inhibitor, MCC950, prevents caspase-4/5-dependent IL-1β production elicited by intracellular LPS. Given that caspase-4 and caspase-5 can both detect cytoplasmic LPS, it is possible that these proteins exhibit some degree of redundancy. Therefore, using CRISPR-based technology, we generated human monocytic cell lines in which caspase-4 and caspase-5 were genetically deleted either individually or together. We found that the deletion of caspase-4, but not caspase-5 suppressed cell death and IL-1β production upon transfection of LPS into the monocyte cytoplasm, whereas deletion of either caspase inhibited both of these phenotypes following infection of cells with Salmonella Typhimurium. Furthermore, double deletion of caspase-4 and -5 had a synergistic effect in the context of Salmonella infection. Our results identify the NLRP3 inflammasome as the specific platform for IL-1β maturation, downstream of cytoplasmic LPS detection by caspase-4/5. We also show that both caspase-4 and caspase-5 are functionally important for human monocytes to respond to an invasive, gram-negative bacterial pathogen.