Mitogen-activated protein kinase (MAPK) cascades play pivotal roles in mediating biotic and abiotic stress responses. Cotton (Gossypium hirsutum) is the most important textile crop in the world, and often encounters abiotic stress during its growth seasons. In this study, a gene encoding a mitogen-activated protein kinase (MAPK) was isolated from cotton, and designated as GhMPK17. The open reading frame (ORF) of GhMPK17 gene is 1494 bp in length and encodes a protein with 497 amino acids. Quantitative RT-PCR analysis indicated that GhMPK17 expression was up-regulated in cotton under NaCl, mannitol and ABA treatments. The transgenic Arabidopsis plants expressing GhMPK17 gene showed higher seed germination, root elongation and cotyledon greening/expansion rates than those of the wild type on MS medium containing NaCl, mannitol and exogenous ABA, suggesting that overexpression of GhMPK17 in Arabidopsis increased plant ABA-insensitivity, and enhanced plant tolerance to salt and osmotic stresses. Furthermore, overexpression of GhMPK17 in Arabidopsis reduced H2O2 level and altered expression of ABA- and abiotic stress-related genes in the transgenic plants. Collectively, these data suggested that GhMPK17 gene may be involved in plant response to high salinity and osmotic stresses and ABA signaling.