A galactomannan was isolated from seeds of a leguminous plant, Desmanthus illinoensis, which is grown in Okinawa, Japan. D-Galactose (molar ratio, 1.0) and D-mannose (0.82) were identified via High-performance Anion Exchange Chromatography Coupled with a Pulse Amperometric Detector. The molecular mass and specific rotation were estimated to be 1000 kDa and +53.8°, respectively. The infrared spectrum indicated that the galactomannan was involved in both α- and β-linkages, and two types of α-linkages were detected at 814 and 830 cm^-1. The ¹H- and ¹³C-NMR spectra indicated that the majority of the β-D-mannan main chain was substituted with mono α-D-galactose or α-D-galacto-disaccharide-side chains. Methylation analysis was used to identify 2,3,4,6-tetra-O-methyl-D-galactose (molar ratio, 3.3), 2,3,4-tri-O-methyl- D-galactose (1.0) and 2,3-di-O-methyl-D-mannose (3.1). Specifically, unique 2,3,4-tri-O-methyl D-galactose residue was identified from mass spectrum. The results suggested that the galactomannan was 1,4-linked-β-D-mannan substituted with α-D-galactose or 1,6-linked-α-D-galacto-disaccharide side chains at C-6 on the main chain. The galactomannan isolated from D. illinoensis was an unusual highly branched polysaccharide, and its chemical structure was proposed. This work is the first to report on the galactomannan involving 1,6-linked α-D-galacto-disaccharide side chains in addition to α-D-galactose mono side chains.