Basic leucine zipper (bZIP) proteins play important roles in responding to biotic and abiotic stresses in plants. However, the molecular mechanisms of plant resistance to pathogens remain largely unclear in poplar. The present study isolated a TGACG-binding (TGA) transcription factor, PeTGA1, from Populus euphratica. PeTGA1 belongs to subgroup D of the bZIP family and was localized to the nucleus. To study the role PeTGA1 plays in response to Colletotrichum gloeosporioides, transgenic triploid white poplars overexpressing PeTGA1 were generated. Results showed that poplars with overexpressed PeTGA1 showed a higher effective defense response to C. gloeosporioides than the wild-type plants. A yeast one-hybrid assay and an electrophoretic mobility shift assay revealed that PeTGA1 could directly bind to the PeSARD1 (P. euphratica SYSTEMIC ACQUIRED RESISTANCE DEFICIENT 1) promoter, an important regulator for salicylic acid biosynthesis. The transactivation assays indicated that PeTGA1 activated the expression of PeSARD1, and PR1 (PATHOGENESIS-RELATED 1), a SA marker gene involved in SA signaling. Subsequently, we observed that the PeTGA1 overexpression lines showed elevated SA levels, thereby resulting in the increased resistance to C. gloeosporioides. Taken together, our results indicated that PeTGA1 may exert a key role in plant immunity not only by targeting PeSARD1 thus participating in the SA biosynthesis pathway but also by involving in SA signaling via activating the expression of PR1.