Collagen is one of the most abundant proteins in animals and a major component of the extracellular matrix (ECM) in tissues. Besides playing a role as a structural building block of tissues, collagens can modulate the behavior of cells, and their deregulation can promote diseases such as cancer. In tumors, collagens and many other ECM molecules are mainly produced by fibroblasts, and recent evidence points towards a role of tumor-derived collagens in tumor progression and metastasis. In this review, we focus on the newly discovered functions of collagens in cancer. Novel findings have revealed the role of collagens in tumor dormancy and immune evasion, as well as their interplay with cancer cell metabolism. Collagens could serve as prognostic markers for cancer patients, and therapeutic strategies targeting the collagen ECM have the potential to prevent tumor progression and metastasis.