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Nanofertilizers can be used as potential contenders to provide macro- and micronutrients to plants. Specifically, iron-oxide-based engineered nanoparticles are promising candidates for next-generation iron-deficiency fertilizers. Aiming to investigate the effect of Fe2O3 magnetic nanoparticles on plant germination and growth, alfalfa seeds were presoaked by a series of concentrations (0, 50, 100, 150, and 200 mg/L) of Fe2O3 NPs in this study. The germination rate, root length, chlorophyll content, and activity of the antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)) were detected. Further, hydroponic alfalfa young seedlings were treated with 50 mg/L Fe2O3 NPs, with 50 mg/L Fe-EDTA as the control. The growth index, chlorophyll content, metal (Fe, Cu, Zn, and Mn) content, and ferric-chelate reductase activity were detected. The results indicated that the presoaking of Fe2O3 NPs promoted alfalfa seed germination in all treatments. Short-time of Fe2O3 NPs presoaking has not affected chlorophyll and iron content while increasing antioxidase (SOD, POD, and CAT) activities at 50 and 200 mg/L concentrations. Fe2O3 NPs of 50 mg/L have the most beneficiary impact on alfalfa seed germination by presoaking. Fe2O3 NPs of 50 mg/L enhanced the seedlings’ growth and increased chlorophyll content, Fe content, and Mn content of hydroponic alfalfa compared with the control. Moreover, ferric chelate reductase activity was significantly lower in alfalfa roots exposure to Fe2O3 NPs than in the control. The results showed that 50 mg/L Fe2O3 NPs could be used as an appropriate and effective treatment concentration for seed pretreatment and fertilizer application. The growth-promoting effect of Fe2O3 NPs on alfalfa is related to its nanostructured properties. More in-depth research is still needed to understand the growth-promoting mechanism of Fe2O3 NPs and the impact of Fe2O3 NPs on the field environment.