Drought and high salinity are major environmental factors that reduce plant growth and development, leading to loss of plant productivity in agriculture. Under these stress conditions, photosynthesis is greatly suppressed despite the high cellular energy cost of stress response processes. Currently, the process that allows plants to secure the energy required for osmotic stress responses remains elusive. Here, we provide evidence that CBR1, a cytochrome b5 reductase, plays an important role in ATP production in response to NaCl and dehydration stresses. Overexpression and loss of function of CBR1 led to enhanced resistance and sensitivity, respectively, to osmotic stress. Upon exposure to osmotic stress, CBR1 was localized to the endoplasmic reticulum (ER) instead of to mitochondria, where it was localized under normal conditions. Transgenic plants overexpressing ER-targeted CBR1, but not mitochondria-targeted CBR1, showed enhanced resistance to osmotic stress and higher expression of SOS1/2/3 and RD29A/B under osmotic stress. CBR1-ER and CBR1-OX plants, but not CBR1-MT plants, had higher levels of ATP and unsaturated fatty acids under osmotic stress. Moreover, these effects were abrogated by thioridazine and 2-deoxy glucose, inhibitors of β-oxidation and glycolysis, respectively, but not by thiazolidinedione, an inhibitor of the mitochondrial pyruvate transporter. Based on these results, we propose that ER-localized CBR1 triggers ATP production via the production and β-oxidation of polyunsaturated fatty acids under osmotic stress.