Hydrated networks of glycans (polysaccharides)-in the form of cell walls, periplasms or gel-like matrices-are ubiquitously present adjacent to cellular plasma membranes(1-4). Yet, despite their abundance, the function of glycans in the extracellular milieu is largely unknown(5). Here we show that the spatial configuration of glycans controls the phase behaviour of multiphase model lipid membranes: inhomogeneous glycan networks stabilize large lipid domains at the characteristic length scale of the network, whereas homogeneous networks suppress macroscopic lipid phase separation. We also find that glycan-patterned phase separation is thermally reversible thus indicating that the effect is thermodynamic rather than kinetic-and that phase patterning probably results from a preferential interaction of glycans with ordered lipid phases. These findings have implications for membrane-mediated transport processes(6-8), potentially rationalize long-standing observations that differentiate the behaviour of native and model membranes(9-13) and may indicate an intimate coupling between cellular lipidomes and glycomes.