Verticillium wilt, a devastating disease in cotton caused by Verticillium dahliae, reduces cotton quality and yield. Heterotrimeric GTP-binding proteins, consisting of Ga, Gb, and Gc subunits, transducers of receptor signaling, function in a wide range of biological events. However, the function of Ga proteins in the reg-ulation of defense responses in plants is largely unexplored, except for a few reports on model species. In the present study, a cotton G-protein a-subunit-encoding gene (GhGPA) was isolated from Verticillium wilt-resistant Gossypium hirsutum (upland cotton) cv. ND601. GhGPA transcription was up-regulated under V. dahliae stress, with higher expression in tolerant than in susceptible cotton cultivars. Subcellular localization revealed GhGPA to be located in the plasma membrane. GhGPA shows high (85.0％) identity with Arabidopsis AT2G26300 (AtGPA1), and AtGPA1 gpa1-4 mutants displayed susceptibil-ity to V. dahliae. Ectopic expression of GhGPA successfully restored the resistance of Arabidopsis gpa1-4 mutants to Verticillium wilt and made them more resistant than the wild type. Overexpression of GhGPA in Arabidopsis markedly increased the resistance and resulted in dramatic up-regulation of pathogenesis-related (PR) genes and increased in H2O2 accumulation and salicylic acid (SA) and jasmonic acid (JA) contents. However, suppressing GhGPA expression via virus-induced gene silencing (VIGS) increased susceptibility to Verticillium wilt, down-regulated the expression of PR and marker genes in SA and JA signaling pathways, and reduced H2O2 content. The contents of SA and JA in Arabidopsis gpa1-4 and VIGS cotton were lower than those in the wild type and empty-vector control. However, GhGPA-overexpressing Arabidopsis contained more SA and JA than the wild type when inoculated with V. dahliae. Thus, GhGPA plays a vital role in Verticillium wilt resistance by inducing SA and JA signaling pathways and regulating the production of reactive oxygen species. These findings not only broaden our knowledge about the biological role of GhGPA, but also shed light on the defense mechanisms involv-ing GhGPA against V. dahliae in cotton.