Significance In mammals, the brain is the organ with the highest level of sialic acids, a family of negatively charged monosaccharides that are commonly expressed as outer terminal residues of cell-surface glycans. Brain sialoglycans play essential roles in brain development, cognition, and disease progression; however, in vivo visualization of the sialoglycan biosynthesis in the mouse brain has been impossible. Here, we introduce a liposome-assisted bioorthogonal reporter (LABOR) strategy for metabolic labeling and visualization of brain sialoglycans in living mice. Applying LABOR, we visualized the biosynthesis of brain sialoglycans by in vivo fluorescence imaging and histological analysis, and identified important sialylated glycoproteins in the brain by glycoproteomics. We discovered that the turnover of sialoglycans is spatially regulated in distinct brain regions.