Background: B-Cell CLL/Lymphoma 9 (BCL9) is a recently described oncogene that promotes tumorigenesis via activation of the Wnt/β-Catenin signaling cascade. Though constitutively active Wnt/β-Catenin signaling is a molecular hallmark of adrenocortical carcinoma (ACC), a potential role for BCL9 to promote Wnt/β-Catenin pathway dysregulation in adrenocortical tumorigenesis remains to be elucidated. Study Design: This study involved a retrospective analysis at a tertiary academic referral center of 27 patients with adrenocortical tumors, including in vitro investigation of BCL9. The Wnt signaling pathway polymerase chain reaction (PCR) array analysis queried comparative mRNA expression profiles of canonical Wnt pathway components including BCL9. Real-time quantitative PCR determined BCL9 mRNA expression levels in tumor samples. Expression levels of BCL9 mRNA were evaluated for correlation with tumor characteristics. RNA interference (RNAi) gene silencing was performed in ACC cell lines SW-13 and NCI-H295R to test the role of BCL9 on clonal cell growth. Results: Expression levels of the BCL9 gene were found to be significantly elevated in ACC compared with normal adrenal tissue (p < 0.05). Furthermore, a significant correlation was observed between BCL9 mRNA levels and the malignant status of adrenocortical tumors (p < 0.05). RNAi gene silencing of BCL9 inhibited clonal cell growth of SW-13 cells (p < 0.05), but not NCI-H295R cells, which carry a constitutively active β-Catenin mutation. Conclusions: The gene BCL9 is overexpressed in malignant adrenocortical tumors and promotes clonal ACC cell growth. These findings suggest that BCL9 overexpression may serve as an alternative driver of constitutive Wnt/β-Catenin activation in ACC and could represent a potential molecular and diagnostic marker of tumor malignancy.