Microelectronic assemblies that include a lithographically-defined substrate integrated waveguide (SIW) component, and related devices and methods, are disclosed herein. In some embodiments, a microelectronic assembly may include a package substrate portion having a first face and an opposing second face; and an SIW component that may include a first conductive layer on the first face of the package substrate portion, a dielectric layer on the first conductive layer, a second conductive layer on the dielectric layer, and a first conductive sidewall and an opposing second conductive sidewall in the dielectric layer, wherein the first and second conductive sidewalls are continuous structures.