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De novo molecular design has yielded proteins and peptides with structures and functions beyond those found in nature. Despite the potential for glycans to form a broader scope of well-defined tertiary architectures, owing to the numerous conjugation sites and stereocenters, no one has yet built glycans with targeted structures and functions from scratch. Here, we designed glycan sequences that fold into programmable 3D architectures. Starting from first-principles, we create a linear glycan that spontaneously adopts a rigid tertiary structure not reported for natural glycans. Considering stereochemical and spatial orientation, we identify a rigid trisaccharide turn unit that programs backbone directionality, driving folding into antiparallel geometry. The combination of this turn unit with multiple cellulose-like strands completes our design, stabilizing a tertiary sheet-like folding, as confirmed by nuclear magnetic resonance spectroscopy and small-angle X-ray scattering (SAXS). To quantitatively evaluate the conformational landscape of our glycans in aqueous solution, we built a semiautomated protocol that integrates SAXS data with molecular dynamics simulations, demonstrating further the effectiveness of our design principles. This is an important step to design and control conformation populations, not just single structures in the solid state or of unknown prevalence in the solution phase. Together, these results show that glycans can be programmed to adopt rigid tertiary structures on demand, opening new avenues for de novo glycan-based architectures in synthetic glycobiology, catalysis, and materials science.