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 was investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and differential pulse voltammetry (DPV) techniques. This electrode exhibited a low detection limit (LOD) of 0.12 μM in the 5–1000 μM range, determined by the equation LOD = 3.3σ/S. The electrochemical response was influenced by key parameters including electrode composition (optimal at 10 % modifier), pH (optimal at pH 7.0), and scan rate, which followed diffusion-controlled kinetics. The oxidation process of nitrite was found to proceed via a two-electron, one-proton mechanism. Selectivity tests demonstrated minimal interference from common ions, while real sample analyses in tap and stream water yielded recovery rates between 99.6 % and 104.1 %, with relative standard deviations (RSD) ranging from 1.6 % to 3.1 %. These results demonstrate the sensor's robustness, environmental compatibility, and applicability for real-world nitrite monitoring.