Drought stress is one of the major constraints limiting crop productivity, primarily through oxidative damage, pigment degradation, and metabolic imbalance. Nanostructured selenium particles (SeNPs) have recently attracted attention for their potential to enhance plant tolerance to abiotic stress. In this study, green-synthesized SeNPs, with a main hydrodynamic size distribution in the range of 90–100 nm, were foliar applied to broccoli (Brassica oleracea var. italica) plants grown under well-watered (100% water holding capacity) and drought (50% water holding capacity) conditions at concentrations of 0, 10, 20 and 50 ppm. Drought stress significantly decreased chlorophyll a and b, total chlorophyll, and carotenoids, while increasing malondialdehyde (MDA) and proline levels, confirming oxidative stress and membrane damage. SeNPs treatments partially mitigated these effects by enhancing pigment stability, increasing carotenoid content, and reducing both MDA and proline accumulation. Phenolic and flavonoid responses exhibited a dose-dependent pattern with the highest stimulation at 50 ppm under drought and moderate enhancement at 10 ppm under optimal irrigation. Antioxidant capacity assays demonstrated that SeNPs modulate plant redox metabolism, in a context-dependent manner, particularly under water deficit. Peroxidase (POD) activity was also significantly induced under drought stress, mainly at 20 ppm. These results indicate that foliar-applied SeNPs can influence physiological and biochemical responses associated with drought tolerance in broccoli. The observed effects are consistent with nanoparticle–leaf surface interactions contributing to redox regulation and stress adaptation, rather than implying direct nanoparticle internalization.