Abstract: Objective To investigate the expression of ribosomal protein autoantibody (tripartite motif-containing 21, TRIM21) in colorectal cancer (CRC) and its effects on the proliferation, migration, and invasion capabilities of CRC cells. Methods The expression of TRIM21 in CRC tissues and adjacent non-cancerous tissues was analyzed using the Clinical Proteomic Tumor Analysis Consortium (CPTAC) database and immunohistochemistry. Fisher’s exact test was used to investigate the correlation between TRIM21 expression and clinical pathological features in CRC. Kaplan-Meier survival analysis and the Log-rank test were employed to explore the relationship between TRIM21 expression and prognosis in CRC patients. Two plasmids (NC plasmid and TRIM21 overexpression plasmid) and small interfering RNAs (siCtrl, siTRIM21-1, and siTRIM21-2) were used to transfect two CRC cell lines, HCT-116 and LoVo, to construct TRIM21 overexpression and knockdown CRC cell lines. The effects of TRIM21 on CRC cell proliferation were analyzed using the CCK-8 and colony formation assays. Western blot was used to examine the impact of TRIM21 on the expression of cell cycle-related proteins in CRC cells. Transwell assays were performed to analyze the effect of TRIM21 overexpression and knockdown on the migration and invasion abilities of the two CRC cell lines. Western blot was also used to detect the impact of TRIM21 overexpression and knockdown on epithelial-mesenchymal transition (EMT) related proteins. Results Compared with adjacent non-cancerous tissues, the expression of TRIM21 was significantly downregulated in CRC tissues. The reduced expression of TRIM21 was significantly associated with poor clinical pathological features and decreased overall survival (OS) and disease-specific survival (DSS) (P<0.001). Compared with the NC group, the TRIM21 overexpression group showed reduced proliferation, migration, and invasion abilities of CRC cells, with decreased levels of cyclin E2 and cyclin D1, increased p21 levels, decreased EMT-related proteins N-cadherin and Snail, and elevated E-cadherin protein levels. Compared with the siCtrl group, the siTRIM21 group exhibited significantly increased proliferation, migration, and invasion capabilities in CRC cells, with elevated levels of cyclin E2 and cyclin D1, reduced p21 levels, increased levels of EMT-related proteins N-cadherin and Snail, and reduced E-cadherin protein levels (P<0.05). Conclusions TRIM21 is downregulated in CRC tissues and can inhibit the proliferation, migration, and invasion abilities of CRC cells. Decreases TRIM21 expression is associated with poor clinical pathological features and reduced survival periods.