The present invention aims at providing a piezoelectric substrate with high joining strength between an electrode of the piezoelectric substrate, particularly an electrode for a common electrode and the outside, 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 21b, a plurality of first electrodes 24 and a second electrode 28 disposed on one main surface of the piezoelectric ceramic layer 21b, a third electrode 24 disposed on the other main surface of the piezoelectric ceramic layer 21b so as to oppose to the first electrodes 25, and a through-conductor 34 electrically connecting the second electrode 28 and the third electrode 24. The second electrode 28 includes a connection portion 28a connected to the through-conductor 3, and a small-width portion 28b having a smaller width than the connection portion 28a when the piezoelectric substrate is viewed from above. The second electrode 28 and the outside are electrically connected to each other at the small-width portion 28b.