Green synthesis of nanoparticles provides an alternative to conventional methods, which often involve harsh conditions and hazardous chemicals. This study utilized Tithonia diversifolia, an invasive plant species in Sri Lanka. Rich in phytochemicals, it is a promising candidate for green synthesis while converting it into a useful application. CuSO₄·5H₂O (Copper (II) sulfate) was used as the metal precursor. This research compared the petals and leaves of T. diversifolia to determine the optimal source for green synthesis of copper oxide (CuO) nanoparticles. Plant extracts (dried, ground, and sieved petals and leaves) were prepared by dissolving the powders in distilled water [1:10 (w/v)], heating at 70 °C for 15 minutes, and then centrifuging at 10,000 rpm. The supernatant was mixed with CuSO₄·5H₂O (0.5 M) and distilled water in a 1:1:1 ratio, heated to 70 °C for 15 minutes, and stirred at room temperature. The colour change indicated nanoparticle production. The solution was centrifuged at 10,000 rpm, washed three times with distilled water, dried at 60 °C, and characterized. Results for petals and leaves were as follows: UV–Vis absorbances at 200–250 nm and 265–285 nm; SEM revealed spherical shapes (20-50 nm) and flake-like structures (~100 nm); FTIR confirmed Cu–O bonding at 500–650 cm⁻¹ with phytochemical residues; zeta potential values were -0.2 mV (petals) and -1.8 mV (leaves), with hydrodynamic diameters of 262 nm and 385 nm, respectively. The study confirms that both are effective, with petals showing improved characteristics. Further research on purification and applications is recommended.