Background/Aim: Casein kinase 2 (CK2) is involved in multiple cellular processes. Furthermore, its overexpression in several human cancers has been associated with tumor progression. In this study, we evaluated the efficacy of the CK2 inhibitor, CX-4945, in gastric cancer cell lines and explored the potential predictive biomarkers for CX-4945 sensitivity. Materials and Methods: The sensitivity to CX-4945 was screened in 49 gastric cancer cell lines by the MTT assay. The mRNA and protein expression of CK2 subunits (α and α’) were determined using qRT-PCR and western blot. Furthermore, the activity of CK2α was measured by ELISA. Gene expression and mutations were analyzed via whole-exome and RNA sequencing. Results: The sensitivity to CX-4945 was determined by the inhibition rate (%) at the effective dose (10 μM) which ranged from –1% to 89% in 49 gastric cancer cell lines. CK2α’, but not CK2α, mRNA expression was correlated with CX-4945 sensitivity. Conclusion: In this study, CX-4945 showed modest antitumor efficacy in gastric cancer cell lines. CK2 might represent a potential therapeutic target for gastric cancer.