Aberrant activation of β-catenin in the nucleus has been implicated in a variety of human cancers, but the fate of nuclear β-catenin is unknown. Here we demonstrate that the tripartite motif-containing protein 33 (TRIM33), acting as an E3 ubiquitin ligase, reduces the abundance of nuclear β-catenin protein. TRIM33-mediated β-catenin is destabilized and is GSK-3β or β-TrCP independent. TRIM33 interacts with and ubiquitylates nuclear β-catenin. Moreover, protein kinase Cδ, which directly phosphorylates β-catenin at Ser715, is required for the TRIM33-β-catenin interaction. The function of TRIM33 in suppressing tumour cell proliferation and brain tumour development depends on TRIM33-promoted β-catenin degradation. In human glioblastoma specimens, endogenous TRIM33 levels are inversely correlated with β-catenin. In summary, our findings identify TRIM33 as a tumour suppressor that can abolish tumour cell proliferation and tumorigenesis by degrading nuclear β-catenin. This work suggests a new therapeutic strategy against human cancers caused by aberrant activation of β-catenin.