Abstract: The pollution of agricultural systems by rare earth elements has become one of the most serious environmental deterioration problems worldwide. The signaling molecule nitric oxide (NO) is involved in the plant response to heavy metals. The aim of this study was to investigate the regulating effect of exogenous NO on the physiological response of oat seedlings to lanthanum (La) stress. We conducted a hydroponic experiment in which the exogenous NO donor sodium nitroprusside (SNP) was applied to oat seedlings under a 20 mmol/L La³⁺ treatment and plant growth, active oxygen metabolism, and the absorption of minerals were investigated. The root length, plant height, and dry weight of oat seedlings were significantly lower in La³⁺ -stressed seedlings than in control seedlings. Compared with control plants, those treated with La³⁺ showed significantly increased active oxygen (O2·- and H₂O₂) and malondialdehyde (MDA) contents in roots and leaves, decreased activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), and increased ascorbate peroxidase (APX) activity and ascorbic acid (AsA) and glutathione (GSH) contents. Addition of 100 μmol/L SNP to oat seedlings under La³⁺stress significantly alleviated the La³⁺-inhibition of growth. Compared with La³⁺-treated seedlings, those treated with La³⁺ + SNP showed reduced contents of O2·-, H₂O₂, MDA, AsA, and GSH in roots and leaves, and increased activities of SOD, CAT, POD, and APX. Seedlings under La³⁺stress showed increased La and copper (Cu) accumulation in the roots and leaves, but decreased absorption of potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), and manganese (Mn). Compared with La³⁺-treated seedlings, those treated with La³⁺ + SNP showed significantly lower La translocation from roots to leaves, increased absorption of K, Ca, Mg, Fe, increased Cu uptake, lower Zn and Mn contents in roots, and higher Zn and Mn contents in leaves. Consequently, exogenous NO could alleviate the oxidative damage and growth inhibition of oat seedlings under La³⁺ stress by increasing the activities of antioxidant enzymes and altering La translocation and mineral absorption.