The increasing chances of manufactured nanoparticles (NPs) being released into the environment highlight the importance of understanding their interactions with plants, which are the basis of ecosystems. In this study, the phytotoxicity of CeO2 NPs on asparagus lettuce was assessed. Lettuce seeds were treated with CeO2 NPs in a plant agar medium at a wide range of concentrations (0–2000 mg L−1) for 5 days. At high concentrations (≥500 mg L−1), CeO2 NPs altered the activity of superoxide dismutase (SOD), induced lipid peroxidation and cell membrane damage, and inhibited the root growth. The results of X-ray absorption near edge fine structure (XANES) indicate that part of the CeO2 NPs were transformed from Ce(IV) to Ce(III) in the roots. The released Ce3+ may account for the phytotoxicity of CeO2 NPs.