Nanomaterials have been developed for various applications due to their unique features, and anew interest is now growing in their use in the agricultural sector as nano-fertilizers. Among the developed nanomaterials, hydroxyapatite nanoparticles (Nano-HAP) possess a relevant potential to be used as a source of nano-phosphorus fertilizer. Here, we evaluated the effect of these nanoparticles on growth traits, herb and essential oil yields, and histological changes of Italian parsley. The following fertilizer treatments were applied: control, NPK normal solution, Nano-HAP (0.25, 0.50, 1.0 g/L). The results showed that the foliar application of Nano-HAP tended to be comparable (and even superior) to that of conventional NPK fertilizer. Italian parsley responded positively to Nano-HAP treatments at 0.5 g/L with higher fresh (~1.5 to 3-fold increase) and dry herb yields and higher essential oil yields (10.64−20.97 L/ha) compared with the control and NPK treatments. Conversely, Nano-HAP at a high concentration of 1.0 g/L adversely influenced the growth and yield characteristics of the Italian parsley. In terms of anatomical traits, the Nano-HAP foliar application showed an improvement in the lamina and petiole measurements. Nano-HAP at 0.5 g/L produced the greatest values for the thickness of lamina, phloem tissue, xylem tissue, main vein of leaf lamina and oil gland diameter, as well as for the thickness of phloem and xylem tissues, and pith diameter of leaf petiole. Overall, our findings indicate that Nano-HAP at 0.5 g/L seems to be the most appropriate treatment for Italian parsley production.