The increasing use of europium oxide nanoparticles (Eu2O3 NPs) in various industrial and biomedical applications raises concerns regarding their environmental accumulation and potential phytotoxic effects. This study aimed to investigate the physiological and biochemical responses of maize (Zea mays L.) to Eu2O3 nanoparticle exposure and to evaluate the ameliorative role of biochar in mitigating nanoparticle-induced toxicity. Hydroponically grown maize seedlings were subjected to Eu2O3 stress (Eu2O3-NP200, 200 μM; Eu2O3-NP500, 500 μM) with or without biochar supplementation at two concentrations (1 and 3 g L-1). Plant growth parameters, photosynthetic performance, elemental composition, oxidative stress markers, and antioxidant defense mechanisms were systematically analyzed. Eu2O3 exposure markedly reduced growth rate, water content, net photosynthesis, transpiration rate, and PSII photochemistry (Fv/Fm), while significantly increasing the levels of reactive oxygen species (ROS) and lipid peroxidation. Elemental analysis confirmed elevated Eu accumulation in plant tissues, accompanied by nutrient imbalance. Biochar treatments substantially alleviated these negative effects by decreasing Eu uptake, enhancing K, Ca, and Mg content, restoring gas exchange parameters, and improving chlorophyll fluorescence efficiency. Furthermore, biochar (dose 1 g L−1) significantly attenuated oxidative damage, as evidenced by substantial reductions in H2O2 up to 51% and TBARS levels up to 33%. Enzymatic assays revealed that biochar enhanced activities of SOD, CAT, POX, GST, and APX, and promoted the regulation of the AsA–GSH cycle, thereby reinforcing the antioxidant defense system under stress conditions. Collectively, the findings demonstrate that biochar application effectively counteracts the adverse physiological and biochemical impacts of Eu2O3 nanoparticle stress in maize by limiting metal accumulation, improving photosynthetic capacity, and enhancing antioxidant activity. These results highlight biochar as a promising eco-friendly amendment to mitigate rare earth element nanoparticle-induced phytotoxicity and contribute to sustainable crop production in contaminated environments.