The T-box transcription factor Brachyury, a molecule frequently detected in human cancers but seldom found in normal adult tissue, has recently been characterized as a driver of the epithelial-to-mesenchymal switch of human carcinomas. In the current investigation, we present data demonstrating that in two different human lung carcinoma models expression of Brachyury strongly correlates with increased in vitro resistance to cytotoxic therapies, such as chemotherapy and radiation. We also demonstrate that chemotherapy treatment in vitro selects for tumor cells with high levels of Brachyury and that the degree of resistance to therapy correlates with the level of Brachyury expression. In vitro and in vivo, human lung carcinoma cells with higher levels of Brachyury divide at slower rates than those with lower levels of Brachyury, a phenomenon associated with marked downregulation of cyclin D1, phosphorylated Rb and CDKN1A (p21). Chromatin immunoprecipitation and luciferase reporter assays revealed that Brachyury binds to a half T-box consensus site located within the promoter region of the p21 gene, indicating a potential mechanism for the observed therapeutic resistance associated with Brachyury expression. Finally, we demonstrate that in vivo treatment of tumor xenografts with chemotherapy results in the selective growth of resistant tumors characterized by high levels of Brachyury expression. Altogether, these results suggest that Brachyury expression may attenuate cell cycle progression, enabling tumor cells to become less susceptible to chemotherapy and radiation in human carcinomas.