The Kruppel-like transcription factor KLF6 is a novel tumor-suppressor gene mutated in a significant fraction of human prostate cancer. It is localized to human chromosome 10p14–15, a region that displays frequent loss of heterozygosity in glioblastoma multiforme (GBM). Indeed, mutations of the KLF6 gene have recently been reported in this tumor type. In this study, we report that the expression of KLF6 is attenuated in human GBM when compared with primary astrocytes. Expression of KLF6 in GBM cells reverts their tumorigenicity both in vitro and in vivo, which is correlated with its transactivation of the p21/CIP1/WAF1 promoter. Additionally, KLF6 inhibits cellular transformation induced by several oncogenes (c-sis/PDGF-B, v-src, H-Ras, and EGFR) that are components of signaling cascades implicated in GBM. Our results provide the first evidence of functional tumor suppression by KFL6, and its loss may contribute to glial tumor progression.