MADS-box proteins play vital roles in plant growth and development. However, few studies have addressed the biological functions of MADS-box genes in cucumber. In this study, a MADS-box gene, CsMADS25, was cloned from cucumber (Cucumis sativus L.). The open reading frame (ORF) of CsMADS25 was 810 bp in length and encoded a deduced protein consisting of 269 amino acids with a calculated MW of 30.53 kDa and a theoretical pI of 5.38. Sequence alignment showed that CsMADS25 shared the highest amino acid identity with CmMADS09 from Cucumis melo. Phylogenetic tree analysis indicated that CsMADS25 was clustered with AGL18 proteins with high bootstrap values. qRT-PCR analysis showed that the expression of CsMADS25 was observably regulated by various abiotic stresses and GAtreatments. Overexpression of CsMADS25 resulted in dwarf and small-leaf phenotypes in transgenic Arabidopsis plants, and the leaf index value (leaf length/width ratio) of transgenic plants was dramatically increased compared with that of wild-type (WT) plants. These findings suggest that CsMADS25 might play important roles in various developmental processes and in response to abiotic stress of cucumber.