This study reports the green synthesis of a hydroxyapatite‑zinc oxide (HAp-ZnO) nanocomposite using Onosma mersinana leaf extract (OMLE) and its application as a highly sensitive electrochemical sensor for nitrite ion detection. The structural and surface characteristics of the synthesized nanomaterial were confirmed by XRD, which revealed crystalline phases corresponding to both HAp and ZnO; SEM analysis showed uniform morphology; EDX confirmed elemental composition; and XPS identified chemical states of Zn, O, Ca, C, and P, verifying successful ZnO doping. The HAp-ZnO nanocomposite was incorporated into a carbon paste electrode (CPE) to fabricate the CPE@HAp-ZnO100 sensor for the electrochemical detection of nitrite. The electrochemical behavior of nitrite on the CPE@HAp-ZnO100 electrode...