The amyloid plaques associated with Alzheimer's disease (AD) comprise fibrillar amyloid-beta (A beta) peptides as well as non-protein factors including glycosaminoglycan (GAG) polysaccharides. GAGs affect the kinetics and pathway of A beta self-assembly and can impede fibril clearance; thus, they may be accessory molecules in AD. Here we report the first high-resolution details of GAG-A beta fibril interactions from the perspective of the saccharide. Binding analysis indicated that the GAG proxy heparin has a remarkably high affinity for A beta fibrils with 3-fold cross-sectional symmetry (3Q). Chemical synthesis of a uniformly C-13-labeled octasaccharide heparin analogue enabled magic-angle spinning solid-state NMR of the GAG bound to 3Q fibrils, and measurements of dynamics revealed a tight complex in which all saccharide residues are restrained without undergoing substantial conformational changes. Intramolecular C-13-N-15 dipolar dephasing is consistent with close (<5 angstrom) contact between GAG anomeric position(s) and one or more histidine residues in the fibrils. These data provide a detailed model for the interaction between 3Q-seeded A beta 40 fibrils and a major non-protein component of AD plaques, and they reveal that GAG amyloid interactions display a range of affinities that critically depend on the precise details of the fibril architecture.