Hepatocellular carcinoma (HCC) is believed to arise from tumor-initiating cells (T-ICs), which are responsible for tumor relapse and metastases. Portal vein tumor thrombus (PVTT) is raised from HCC and strongly correlated to a poor prognosis. However, the mechanism underling the formation of PVTT is largely unknown. Herein, we provide evidence that RNA polymerase II subunit 5 (RPB5)-mediating protein (RMP) was progressively upregulated in PVTT and overexpressed RMP appeared to increase T-ICs self-renewal. Moreover, RMP promoted metastases of PVTT cells and HCC cells in vitro and in vivo. Knockdown of RMP attenuated T-ICs self-renewal and reversed epithelial-mesenchymal transition (EMT) in HCC and PVTT cells. The neutralizing assays suggested that interleukin-6 (IL-6) had an indispensable role in RMP regulating metastases and self-renewal of HCC cells. Furthermore, the transcription of IL-6 was verified to be modulated by RMP via interaction with p65 and RPB5, through which expanding the T-IC/cancer stem cell populations, as well as inducing EMT was promoted. These results suggested that RMP may promote PVTT formation by promoting IL-6 transcription. Thus, RMP serves as a potent factor contributed to develop PVTT and a promising therapeutic target for HCC patients.