Galactomannans from the endosperm of mature seeds of Prosopis affinis were isolated following different extraction procedures in yields of up to 64%. The polysaccharides were characterized by HPLC, GC-MS, and NMR spectroscopy, and their physicochemical properties were studied by SEC/MALLS-QUELS/RI and rheological methods. The monosaccharide composition of the extracted galactomannans showed only mannose and galactose in a ~1.5 ratio, similar to galactomannans found in guar gum and obtained from other Prosopis species. The structural characterization showed that these polysaccharides are composed by a b-(1/4)-linked mannose backbone with galactopyranosyl residues attached through a (1/6) linkages. The weight-average molecular weight (Mw) of the P. affinis galactomannans went from (1.6 ± 0.2) x 106 to (1.8 ± 0.1) x 106 g/mol, and the intrinsic viscosity, in the 9.9 to 10.7 dL/g range, was comparable to that reported for other extracts from the Prosopis genre. Although the molecular weight distribution and intrinsic viscosity were dependent on the extraction procedure, the mannose-togalactose ratio was almost invariant. Conformation plots showed that the polymer extracted at higher temperatures is more cohesive, a characteristic that could be attributed to a greater number of polymerpolymer interactions. Our findings show that the P. affinis galactomannan has nearly monomodal molecular weight distribution, functional properties similar to those of guar gum, and can be isolated from an abundant and readily available non-traditional source. It therefore represents a promising new biomaterial for the food and pharmaceutical industries that can be produced from the regional native flora in a sustainable way.