The purpose of the present invention is to provide: a piezoelectric substrate wherein the bonding strength between an electrode of the piezoelectric substrate, particularly an electrode for a common electrode and an external member is high; an assembly which uses this piezoelectric substrate; a liquid ejection head; and a recording device. A piezoelectric substrate according to the present invention is a plate-like piezoelectric substrate which comprises: a piezoelectric ceramic layer (21b); a plurality of first electrodes (25) and a second electrode (28), which are arranged on one main surface of the piezoelectric ceramic layer (21b); a third electrode (24) that is arranged on the other main surface of the piezoelectric ceramic layer (21b) so as to face the plurality of first electrodes (25); and a through conductor (34) that electrically connects the second electrode (28) and the third electrode (24) with each other. This piezoelectric substrate is characterized in that: when the piezoelectric substrate is viewed in plan, the second electrode (28) comprises a connection part (28a) that is connected to the through conductor (34) and a narrow part (28b) that is smaller in width than the connection part (28a); and the second electrode (28) is electrically connected to an external member at the narrow part (28b).