Superionic conductors with ionic conductivity on the order of mS cm-1 are expected to revolutionize the development of solid-state batteries (SSBs). However, currently available superionic conductors are limited to only a few structural families such as garnet oxides and sulfide-based glass/ceramic. Interfaces in composite systems such as alumina in lithium iodide have long been identified as a viable ionic conduction channel, but practical superionic conductors employing the interfacial conduction mechanism are yet to be realized. Here we report a novel method that creates continuous interfaces in the bulk of composite thin films. Ions can conduct through the interface, and consequently, the inorganic phase can be ionically insulating in this type of bulk interface superionic conductors (BISCs). Ionic conductivities of lithium, sodium, and magnesium ion BISCs have reached 1.16 mS cm-1, 0.40 mS cm-1, and 0.23 mS cm-1 at 25 °C in 25 μm thick films, corresponding to areal conductance as high as 464 mS cm-2, 160 mS cm-2, and 92 mS cm-2, respectively. Ultralow overpotential and stable long-term cycling for up to 5000 h were obtained for solid-state Li metal symmetric batteries employing Li ion BISCs. This work opens new structural space for superionic conductors and urges for future investigations on detailed conduction mechanisms and material design principles.