on parsley physiology and antioxidant responses. RKE exhibited high phenolic (520.4 mg GAEg−1) and flavonoid (82.4 mg QE g−1) contents, with strong dose-dependent antioxidant activity (up to 82.8% at 1000 µg mL−1). HPLC analysis of RKE revealed major phenolics, including chlorogenic and gallic acids. A visible color shift to ruby red initially indicated the formation of Se-NPs + RKE. It was further supported by TEM, DLS analysis, and UV–Vis spectroscopy, showing an absorption peak shift from 380 to 395 nm upon conjugation. On the other hand, a field experiment on parsley was conducted using five treatments (control, RKE 500 µg mL−1, RKE 1000 µg mL−1, Se-NPs + RKE 500 µg mL−1, and Se-NPs + RKE 1000 µg mL−1), and the results showed that both RKE and Se-NPs + RKE significantly enhanced parsley growth compared to the control. RKE at 1000 µg mL−1 produced the tallest plants (41 cm) and highest fresh weight (4.52 g), relative to 31 cm and 1.20 g in controls. Se-NPs + RKE at 500 µg mL−1 also promoted growth (39 cm; 2.77 g), although higher doses reduced plant height. Photosynthetic pigments were markedly improved, with chlorophyll a peaking at 2.38 mg g−1 under RKE 1000 µg mL−1, while carotenoids were maximized at 0.70 mg g−1 with Se-NPs + RKE 500 µg mL−1. Antioxidant enzymes (catalase, peroxidase, SOD) were significantly elevated, reaching up to 33.61 units for catalase under Se-NPs + RKE 1000 µg mL−1. Proline content also increased, with RKE treatments reaching \ ~ 0.42 mg g−1 FW and Se-NPs + RKE up to 0.90 mg g−1 FW. These results highlight the potential of RKE and Se-NPs + RKE as eco-friendly biostimulants for enhancing parsley productivity, supporting sustainable agriculture. Nevertheless, the dose-dependent responses, particularly with Se NPs + RKE, underline the need for careful optimization to minimize potential toxicity at higher concentrations.