It is believed that hundreds of genes, including photosynthesis-related genes, are typically involved in plant response to aphid feeding. Up to now, however, it is little known on the relationship between the photosynthesis-related genes and plant response to herbivores. In this study, we identified a cotton photosynthesis-related gene (GhPSAK1) which belongs to PSI-PSAK family and encodes a putative protein of 162 amino acids. RT-PCR analysis revealed that GhPSAK1 transcripts in leaves were increased at 12–24 h, but decreased at 48–72 h after cotton aphid attack or wounding induction. Choice assay and no-choice assay demonstrated that overexpression of GhPSAK1 in Arabidopsis improved plant tolerance to green peach aphids (Myzus persicae). The defense response genes related to salicylic acid signaling pathway were enhanced in the GhPSAK1 overexpressing transgenic plants. In addition, the callose amount in transgenic Arabidopsis leaves was more than that of wild type. Contents of the soluble sugars and total amino acids were also altered in leaves of transgenic Arabidopsis plants. Activities of superoxide dismutase and peroxidase in transgenic leaves were higher than those of wild type. These results suggested that GhPSAK1 may be involved in regulation of cotton response and tolerance to aphid attack.