Herein, we report a unique single-step biogenic deposition precipitation (BDP) approach as a straightforward route for producing efficient Schottky contact between noble metal nanoparticles and wide-band-gap semiconductors. Successful preparation of Ag–ZnO heterojunctions using fennel seed extract (FSE) has been described. The effective biomolecules available in the seeds functioned as novel biogenic materials for the precipitation of adsorbed silver ions (Ag⁺) on ZnO particles as metallic silver (Ag⁰). The as-prepared composite materials were characterized using diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), energy dispersive X-ray (EDX) study, X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL) and high-resolution transmission electron microscopy (HR-TEM). The biogenically prepared Ag–ZnO nanocomposites exhibited excellent photocatalytic activity towards reduction/degradation of colored (rhodamine B (Rh-B) dye) as well as colorless (chlorpyrifos pesticide) pollutants when irradiated under solar light. Among the prepared photocatalysts, 3 wt% Ag–ZnO hybrid composite showed the best photocatalytic activity by efficiently degrading hazardous organic pollutants within a very short time. The superior photocatalytic performance of biogenically prepared Ag–ZnO heterojunctions can be ascribed to the clean production of steady and efficient Schottky contact between plasmonic AgNPs and semiconducting ZnO NPs.