Inflammasomes are multiprotein signaling complexes that activate the innate immune system. Canonical inflammasomes recruit and activate caspase-1, which then cleaves and activates IL-1β and IL-18, as well as gasdermin D (GSDMD) to induce pyroptosis. In contrast, non-canonical inflammasomes, caspases-4/-5 (CASP4/5) in humans and caspase-11 (CASP11) in mice, are known to cleave GSDMD, but their role in direct processing of other substrates besides GSDMD has remained unknown. Here, we show that CASP4/5 but not CASP11 can directly cleave and activate IL-18. However, CASP4/5/11 can all cleave IL-1β to generate a 27-kDa fragment that deactivates IL-1β signaling. Mechanistically, we demonstrate that the sequence identity of the tetrapeptide sequence adjacent to the caspase cleavage site regulates IL-18 and IL-1β recruitment and activation. Altogether, we have identified new substrates of the non-canonical inflammasomes and reveal key mechanistic details regulating inflammation that may aid in developing new therapeutics for immune-related disorders. Display omitted •Human CASP4/5 directly process IL-18 at D36 to generate the activated cytokine•CASP4/5 cleave IL-1β at D27 into a p27 species that inactivates downstream IL-1R signaling•The tetrapeptide sequence of IL-18 and IL-1β regulates their processing by caspases•CASP11 can process IL-1β into the deactivating p27 species but does not process IL-18 Exconde et al. report that the non-canonical inflammasomes directly process IL-18 and IL-1β. The tetrapeptide sequence regulates IL-18 and IL-1β processing to generate an active or inactive cytokine respectively. These results suggest that the non-canonical inflammasomes directly modulate inflammation and may have a broader substrate repertoire than previously known.