Alternative polyadenylation of mRNA has important but poorly understood roles in development and cancer. Activating mutations in the Ras oncogene are common drivers of many human cancers. From a screen for enhancers of activated Ras (let-60) in Caenorhabditis elegans, we identified cfim-1, a subunit of the alternative polyadenylation machinery. Ablation of cfim-1 increased penetrance of the multivulva phenotype in let-60/Ras gain-of-function (gf) mutants. Depletion of the human cfim-1 ortholog CFIm25/NUDT21 in cancer cells with KRAS mutations increased their migration and stimulated an epithelial-to-mesenchymal transition. CFIm25-depleted cells and cfim-1 mutants displayed biased placement of poly(A) tails to more proximal sites in many conserved transcripts. Functional analysis of these transcripts identified the multidrug resistance protein mrp-5/ABCC1 as a previously unidentified regulator of C. elegans vulva development and cell migration in human cells through alternative 3′UTR usage. Our observations demonstrate a conserved functional role for alternative polyadenylation in oncogenic Ras function.