CarNAC3 contains 285 amino acids and a conserved NAC domain. NAC genes, including NAM, ATAF1, ATAF2, and CUC2, are members of one of the largest transcription factor families in plants. CarNAC3, a member of the NAP group, plays an important role in plant development and responses to abiotic stresses. In this study, CarNAC3 was transformed into hybrid poplar plants (Populus deltoides x P. euramericana 'Nanlin895') using Agrobacterium tumefaciens. PCR analysis confirmed integration of the introduced T-DNA into the target genome. Reverse transcription PCR confirmed the transformation, and Southern and northern blotting verified the transgene copy number and gene expression, respectively. Fourteen lines of positive transformants were transplanted into a greenhouse to verify their drought and salt tolerances. Under normal conditions, transgenic poplar plants were shorter than the wild-type, but under drought and salt stresses, they maintained their normal rooting and stem growth rates, while those of the wild-type plants were suppressed. Under stress conditions, CarNAC3 expression caused increases in proline and photosynthetic pigment levels and in antioxidant enzyme activities. Furthermore, the expression of CarNAC3 lowered malondialdehyde concentrations compared with the wild-type control. Overall, our results indicated that the CarNAC3 transgene enhanced drought and salt tolerance in transgenic poplar plants.