Mycobacteria are surrounded by a dynamic, glycolipid-rich envelope that controls both virulence and immune recognition, yet how cytosolic lectins detect and interpret the organization of surface glycans during intracellular infection remains unclear. Here, we show that Dictyostelium discoideum discoidins act as cytosolic sensors of mycobacterial envelope organization. During Mycobacterium marinum infection, Discoidin A and Discoidin E assemble into foci on intracellular bacilli and bind surface-associated glycan patches. Using transposon mutagenesis, synthetic mycobacterial glycan arrays, biochemical fractionation and targeted envelope mutants, we find that discoidins recognize a restricted set of lipid-linked glycans enriched in methylated L-rhamnose motifs, including structures associated with LOS, PGL and GPL. Binding depends on the H-type lectin β-galactoside-binding pocket and is inhibited by point mutations and TDG, a soluble disaccharide competitor, demonstrating glycan-dependent recognition. Specific assays led to exclusion of major structural carbohydrates, including AG-PG, AM/LAM, α-glucan and TDM, while protease treatment of capsular material and fractionation of polar lipids, identified both GPL and LOS-like glycolipids as dominant discoidin ligands. Disruption of LOS biosynthesis or PDIM/PGL-dependent envelope organization reduced discoidin binding to intact bacteria even though some ligands remained detectable by dot blot in various envelope extracts. Thus, discoidins do not simply detect ligand abundance, but binding depends on envelope perturbations and unmasking of glycolipids and glycopeptidolipids. These findings reveal pathogen envelope glycans as direct targets of cytosolic lectin surveillance and establish discoidins as probes of mycobacterial envelope remodelling during intracellular infection.