Oncogenic KRAS contributes to malignant transformation, antiapoptosis, and metastasis in multiple human cancers, such as lung, colon, and pancreatic cancers and melanoma. MicroRNAs (miRNAs) are endogenous 18- to 25-nucleotide noncoding small RNAs that regulate gene expression in a sequence-specific manner via the degradation of target mRNAs or inhibition of protein translation. In the present study, using array-based miRNA profiling in IMR90 and MCF10A cells expressing oncogenic KRAS, we identified that the expression of the microRNA 200 (mir-200) family was suppressed by KRAS activation and that this suppression was mediated by the transcription factors JUN and SP1 in addition to ZEB1. Restoration of mir-200 expression compromised KRAS-induced cellular transformation in vitro and tumor formation in vivo. In addition, we found that enforced expression of mir-200 abrogated KRAS-induced resistance to apoptosis by directly targeting the antiapoptotic gene BCL2. Finally, mir-200 was able to antagonize the epithelial-mesenchymal transition (EMT) driven by mutant KRAS. Collectively, our results suggest that repression of endogenous mir-200 expression is one of the important cellular responses to KRAS activation during tumor initiation and progression.