Missense mutations in exon 3 of , the gene encoding β-catenin, are associated with poor outcomes in endometrial carcinomas (EC). Clinically, mutation status has been difficult to use as a predictive biomarker as β-catenin oncogenic activity is modified by other factors, and these determinants are unknown. Here we reveal that CD73 restrains the oncogenic activity of exon 3 β-catenin mutants, and its loss associates with recurrence. Using 7 patient-specific mutants, with genetic deletion or ectopic expression of CD73, we show that CD73 loss increases β-catenin-TCF/LEF transcriptional activity. In cells lacking CD73, membrane levels of mutant β-catenin decreased which corresponded with increased levels of nuclear and chromatin-bound mutant β-catenin. These results suggest CD73 sequesters mutant β-catenin to the membrane to limit its oncogenic activity. Adenosine A1 receptor deletion phenocopied increased β-catenin-TCF/LEF activity seen with deletion, suggesting that the effect of CD73 loss on mutant β-catenin is mediated via attenuation of adenosine receptor signaling. RNA-seq analyses revealed that deletion alone drives pro-tumor Wnt/β-catenin gene expression and, with CD73 loss, β-catenin mutants dysregulate zinc-finger and non-coding RNA gene expression. We identify CD73 as a novel regulator of oncogenic β-catenin and help explain variability in patient outcomes in mutant EC.