Abstract text: β-Mannans are a major class of hemicelluloses ubiquitously distributed in the plant kingdom and show structural diversity depending on plant β-Mannans are a major class of hemicelluloses ubiquitously distributed in the plant kingdom and show structural diversity depending on plant species and tissues. We recently reported that a new type of mannan, called β-galactoglucomannan (β-GGM), is widely distributed in the primary cell walls of eudicots. β-GGM has an alternating Glc-Man unit in the backbone and β-1,2-linked Gal residues on α-1,6-linked Gal side chains as a common feature; however, no structural variation has been reported. Here, we report an unusual structure of β-GGM found in viscin, a sticky and extensible substance from mistletoe (Viscum album) berries. The structure was determined by enzymatic degradation, mass spectrometry, and nuclear magnetic resonance. It was absent in immature berries lacking viscin, indicating a tissue-specific distribution. These results suggest that, similar to the structurally related hemicellulose xyloglucan, β-GGM displays structural and functional diversity throughout the plant kingdom.