The article presents the data on the structure and mechanisms of β-catenin functioning. The basic aspects of the role of β-catenin in malignant transformation have been studied at various tumors. Primary structure of β-catenin allows it to interact with many factors and ligands, including transcription factors, α-catenin, cadherin, Axin, Rho family GTPases, Bcl9 et al. This interaction is the base for β-catenin's intracellular multi-functioning. The review presents data on the participation of β-catenin in the mechanisms of adhesion, regulation of RNA metabolism, formation contacts with the cytoskeleton and its role in the canonical Wnt signaling pathway, marked examples pro-inflammatory and anti-inflammatory effects of β-catenin. The β-catenin involvement in malignant transformation and progression of certain tumors is not in doubt. The data on the changes in β-catenin expression in the given examples of colon cancer, prostate cancer, different forms of thyroid cancer and hepatocellular carcinoma are presented with the prospects of its use as a marker and a predictor of malignant transformation. Continued research in this area will not only make use of β-catenin as a potential predictor of malignant tumors, but also to develop approaches to targeted therapy.