During malignant transformation, glycosylation is heavily altered compared with healthy tissue due to differential expression of glycosyltransferases, glycosidases and monosaccharide transporters within the cancer microenvironment. One key change of malignant tissue glycosylation is the alteration of sialic acid processing that leads to a general upregulation of sialylated glycans (hypersialylation) on cell surfaces and an increased introduction of the non-human sialic acid N-glycolyl-neuraminic acid (Neu5Gc) instead of N-acetyl-neuraminic acid into cell surface glycans. These changes have been shown to be the result of altered sialyltransferase and sialidase expression. Functionally, cancer-associated hypersialylation appears to directly impact tumor cell interaction with the microenvironment, in particular the modulation of sialic acid-binding lectins on immune cells. Moreover, Neu5Gc expression in human tissues enhances inflammation due to an anti-Neu5Gc immune response, which can potentially influence inflammation-induced cancer and cancer-associated inflammation. In this review, we summarize the changes of sialic acid biology within the malignant microenvironment and the resulting effect on cancer immunity.