The transcription factor SNAIL1 is a master regulator of epithelial-to-mesenchymal transition, a process entailing massive gene expression changes. To better understand SNAIL1-induced transcriptional reprogramming we performed time-resolved transcriptome analysis upon conditional SNAIL1 expression in colorectal cancer cells. Gene set variation analyses indicated that SNAIL1 strongly affected features related to cell cycle and Wnt/β-Catenin signalling. This correlated with upregulation of LEF1, a nuclear binding partner of β-Catenin. Likewise, transcriptomes of cell lines and colorectal cancers, including poor-prognosis mesenchymal tumours, exhibit positively correlated SNAI1 and LEF1 expression, and elevated LEF1 levels parallel increased patient mortality. To delineate the functional contribution of LEF1 to SNAIL1-induced EMT, we used the CRISPR/Cas9 system to knock-out LEF1 in colorectal cancer cells, and to engineer cells that express LEF1 mutants unable to interact with β-Catenin. Both, complete LEF1-deficiency and prevention of the β-Catenin-LEF1 interaction impaired the ability of SNAIL1 to elicit expression of an alternative set of Wnt/β-catenin targets, and to promote cancer cell invasion. Conversely, overexpression of wildtype, but not of mutant LEF1, stimulated alternative Wnt/β-Catenin target gene expression, and caused cell-cycle arrest. Moreover, like SNAIL1, LEF1 retarded tumour growth in xenotransplantations. Thus, LEF1 phenocopies SNAIL1 with respect to several critical aspects of EMT. Indeed, comparative transcriptomics suggested that 35% of SNAIL1-induced transcriptional changes are attributable to LEF1. However, LEF1 did not autonomously induce epithelial-to-mesenchymal transition. Rather, LEF1 appears to be a strictly β-Catenin-dependent downstream effector of SNAIL1. Apparently, SNAIL1 employs β-Catenin-LEF1 complexes to redirect Wnt/β-Catenin pathway activity towards pro-invasive and anti-proliferative gene expression. This article is protected by copyright. All rights reserved.