Significance: Sialic acid (Sia) residues are essential monosaccharides in mammals and confer multiple biological functions. Their precise generation is important for both structure–function studies and biotechnological applications. We describe a unique technology that enables the controlled generation of protein sialylation in Nicotiana benthamiana . The plant engineering approach relies on a combination of endogenous glycan deconstruction and the introduction of human sialylation capabilities. An arrangement of transgenic and transient expression modules resulted in the targeted synthesis of Sia structures in three different linkage types, reaching a polymerization degree exceeding 40 residues (polySia). Importantly, the obtained functional activities of polySia point to novel biotherapeutic applications. Our results highlight the exceptional flexibility of the plant-based expression platform for engineering complex posttranslational protein modifications.