WRKY transcription factors are widely involved in abiotic stress responses in plants. However, their roles in the abiotic stresses of Malus plants are still not well known. In this study, a WRKY gene is isolated from Malus baccata (L.) Borkh. and designated as MbWRKY5. MbWRKY5 contains two WRKY domains and one Cys₂-His₂ (C₂H₂) zinc-finger motif, and was localized in the nucleus. The expression levels of MbWRKY5 were up-regulated by salinity, heat, cold, drought, and abscisic acid treatments in M. baccata seedlings. When MbWRKY5 was introduced into tobacco, an improvement in tolerance to drought and salt was achieved in transgenic plants. Under drought and salt treatments, transgenic plants had higher contents of chlorophyll, proline, glutathione, and ascorbate, and increased activities of peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) than wild-type (WT) tobaccos. Compared with WT plants, overexpression of MbWRKY5 in transgenic tobacco also led to decreased levels of malondialdehyde and hydrogen peroxide (H₂O₂) under drought and salt stresses. Moreover, the MbWRKY5-OE tobaccos increased the expression levels of stress-related genes involved in oxidative stress response (NtPOD, NtSOD and NtCAT) and membrane protection (NtLEA5, NtERD10D, and NtP5CS), especially under drought and salt stresses. These results suggest that the MbWRKY5 gene plays a positive regulatory role in drought and salt stress responses.