The present invention aims at providing a piezoelectric substrate that is less susceptible to the influence of adhesive after being subjected to connection, as well as an assembly, a liquid discharge head, and a recording device, each of which uses the piezoelectric substrate. The piezoelectric substrate of the present invention is a flat plate shaped piezoelectric substrate including a piezoelectric ceramic layer, a plurality of first electrodes, a second electrode, and a third electrode. The second electrode includes an outer peripheral part disposed so as to surround the entirety of the first electrodes, and a protrusion extending from the outer peripheral part toward the inside of the outer peripheral part when the piezoelectric substrate is viewed from above. The second electrode and the third electrode are electrically connected to each other at the protrusion. The outer peripheral part is used for an electrical connection with the outside.