Perturbations in transforming growth factor-beta (TGF-beta) signaling can lead to a plethora of diseases, including cancer. Mutations and posttranslational modifications (PTMs) of the partner of SMAD complexes contribute to the dysregulation of TGF-beta signaling. Here, we reported a PTM of SMAD4, R361 methylation, that was critical for SMAD complexes formation and TGF-beta signaling activation. Through mass spectrometric, co-immunoprecipitation (Co-IP) and immunofluorescent (IF) assays, we found that oncogene protein arginine methyltransferase 5 (PRMT5) interacted with SMAD4 under TGF-beta 1 treatment. Mechanically, PRMT5 triggered SMAD4 methylation at R361 and induced SMAD complexes formation and nuclear import. Furthermore, we emphasized that PRMT5 interacting and methylating SMAD4 was required for TGF-beta 1-induced epithelial-mesenchymal transition (EMT) and colorectal cancer (CRC) metastasis, and SMAD4 R361 mutation diminished PRMT5 and TGF-beta 1-induced metastasis. In addition, highly expressed PRMT5 or high level of SMAD4 R361 methylation indicated worse outcomes in clinical specimens analysis. Collectively, our study highlights the critical interaction of PRMT5 and SMAD4 and the roles of SMAD4 R361 methylation for controlling TGF-beta signaling during metastasis. We provided a new insight for SMAD4 activation. And this study indicated that blocking PRMT5-SMAD4 signaling might be an effective targeting strategy in SMAD4 wild-type CRC.