The NLRP3 inflammasome is an important regulator of inflammation and immunity. It is a multimolecular platform formed within cells that facilitates the activation of proinflammatory caspases to drive secretion of cytokines such as interleukin-1β (IL-1β). Knowledge of the mechanisms regulating formation of the NLRP3 inflammasome is incomplete. Here we report Cl channel-dependent formation of dynamic ASC oligomers and inflammasome specks that remain inactive in the absence of K efflux. Formed after Cl efflux exclusively, ASC specks are NLRP3 dependent, reversible, and inactive, although they further prime inflammatory responses, accelerating and enhancing release of IL-1β in response to a K efflux-inducing stimulus. NEK7 is a specific K sensor and does not associate with NLRP3 under conditions stimulating exclusively Cl efflux, but does after K efflux, activating the complex driving inflammation. Our investigation delivers mechanistic understanding into inflammasome activation and the regulation of inflammatory responses.