Ras-associated protein 1(Rap1) is a member of the RAS family of small G proteins and regulates several signal pathways involved in carcinogenesis. Rap1 consists of two highly homologous isoforms, Rap1a and Rap1b. Increasing data suggest that the deregulated activation of Rap1b is involved in a spectrum of malignancies. Accumulating evidence also indicates effects of Rap1b on cell proliferation, metastasis, angiogenesis, and treatment resistance. Rap1b overexpresses in many tumors and has prognostic values, which are regulated by A2br, miRNAs, and other upstream effectors. This article aims to review research progress in function, significance, and regulation of Rap1b in malignancy.