Cadmium (Cd) is a widely distributed soil contaminant which induces oxidative damage and is therefore toxic to plants. Although selenium oxyanions such as selenite (SeO32-) and selenate (SeO42-) can alleviate Cd stress to plants, it is not known whether selenium nanoparticles (SeNPs) are able to do the same. The present study demonstrated the positive impact of both SeNPs and SeO32- on Brassica napus L. growth under conditions of Cd stress. Underlying mechanisms were elucidated using an oxidative stress detection assay, whole-genome RNA sequencing, and RT-qPCR. Application of selenium, especially in the form of SeNPs, decreased Cd-induced reactive oxygen species production by inhibiting the expression of NADPH oxidases (BnaRBOHC, BnaRBOHD1, and BnaRBOHF1) and glycolate oxidase (BnaGLO), thereby decreasing oxidative protein and membrane lipid damage. In addition, SeNPs improved resistance to Cd stress by decreasing Cd accumulation, maintaining intracellular calcium homeostasis, promoting disulfide bond formation, and restoring the waxy outer layer of the leaf surface. Although both forms of selenium decreased Cd toxicity, the beneficial concentration range was more extensive for SeNPs than for SeO32-. Display omitted •SeNPs alleviated the toxic effects of Cd and improved the plant growth in B. napus.•SeNPs demonstrated superior roles in inhibiting ROS production and scavenging ROS.•SeNPs improved the basal metabolism and maintained calcium homeostasis.•SeNP is an environmental-friendly Se supplement for alleviating Cd stress.