Ovarian cancer is a nearly uniform lethal disease and its highly aggressive metastatic phenotype portends a poor prognosis. Lack of a well-controlled, relevant experimental model has been a major obstacle to identifying key molecules causing metastasis. Here we describe the creation of a new isogenic model of spontaneous human ovarian cancer metastasis exhibiting opposite phenotypes—highly metastatic (HM) and non-metastatic (NM)—both in vitro and in vivo. HM was unique in its ability to metastasize consistently to the peritoneum, mimicking the major dissemination route of human ovarian cancer. In contrast, NM failed to form detectable metastases, although it was equally tumorigenic. Using comparative label-free quantitative liquid chromatography tandem mass spectrometry (LC-MS/MS), we identified β-catenin, which we demonstrated for the first time as having a direct role in the pathogenesis of ovarian cancer metastasis. Our studies also revealed a previously unrecognized role of β-catenin in the downregulation of multiple microRNAs (miRNAs) through attenuating miRNA biogenesis by targeting Dicer, a key component of the miRNA-processing machinery. One such downregulated miRNAs was miR-29s involved in epithelial-to-mesenchymal transition and subsequent stem cell traits. Silencing β-catenin or overexpressing Dicer or miR-29 mimics in HM significantly reduced the ability of these cells to migrate. β-catenin-knockdown cells also failed to metastasize in an orthotopic model of ovarian cancer. Meta-analysis revealed an increase in CTNNB1 and a decrease in DICER1 expression levels in the high-risk group. These results uncover β-catenin as a critical factor in promoting ovarian cancer aggressiveness and a new mechanism linking between β-catenin and miRNA downregulation underlying this process.