We incorporate the concept of tumor metastasis mechanism to establish a database of tumor metastasis mechanism-associated genes. At present, all or the majority of published databases reported metastasis genes based on the concept of using cancer types or hallmarks of cancer/metastasis. Since tumor metastasis is a dynamic process and involving many cellular and molecular processes, those databases cannot provide information on the sequential relations, as well as cellular and molecular mechanism among different metastasis stages. Being motivated by this, we constructed a tumor metastasis mechanism-associated gene (TMMG) database based on using the metastasis mechanism concept. This tumor metastasis mechanism-associated gene database (TMMGdb) contains a wealth of annotations: biological pathways, expressed tissues, upstream miRNAs, genetic mutations, cancer driver genes, drug resistance, transcription factor-mediated protein-protein interaction networks (PPIN), metastasis-associated differentially expressed genes (DEGs) for 14 cancer types and JCR journal ranking information.