This monolithic reactor is an adaptable and scalable, flow-through reaction containment apparatus embodied as a one-piece monolithic block of material that retains re-configurability to improve reaction processing. This apparatus increases operational flexibility, adaptable design, and vastly simplifies construction of tubular reaction-containment configurations. Internally, the monolithic block comprises one or more closely spaced, functional voids which operate as fluid channels that can be configured in various geometric arrangements. The apparatus is widely scalable, provides high thermodynamic efficiency, manufacturing simplicity, and affordability for varied operations through additive manufacturing, and has a compact physical footprint.