Previously, we identified β-galactoside α(2,6)-sialyltransferase (ST6Gal I) as a candidate biomarker for ionizing radiation. The expression of ST6Gal I and the level of protein sialylation increased following radiation exposure in a dose-dependent manner. Radiation induced ST6Gal I cleavage and the cleaved form of ST6Gal I was soluble and secreted. Sialylation of integrin β1, a glycosylated cell surface protein, was stimulated by radiation exposure and this increased its stability. Overexpression of ST6Gal I in SW480 colon cancer cells that initially showed a low level of ST6Gal I expression increased the sialylation of integrin β1 and also increased the stability of the protein. Inhibition of sialylation by transfection with neuraminidase 2 or neuraminidase 3 or by treatment with short interfering RNA targeting ST6Gal I reversed the effects of ST6Gal I overexpression. In addition, ST6Gal I overexpression increased clonogenic survival following radiation exposure and reduced radiation-induced cell death and caspase 3 activation. However, removal of sialic acids by neuraminidase 2 or knockdown of expression by short interfering RNA targeting ST6Gal I restored radiation-induced cell death phenotypes. In conclusion, radiation exposure was found to increase the sialylation of glycoproteins such as integrin β1 by inducing the expression of ST6Gal I, and increased protein sialylation contributed to cellular radiation resistance.