Abstract The hydrocolloid extracted from Caesalpinia ferrea var. ferrea seeds purified by membrane filtration was denominated GMPF (14% yield) and contained 75% total carbohydrate and 9% protein, and the mannose to galactose (Man:Gal) ratio was around to 2.1:1.0. The periodate oxidation, methylation and 1D/2D NMR spectra results indicated galactomannan (GM) with a (1 → 4)-linked β- d-mannopyranose backbone, partially substituted at O-6 with single-unit α- d-galactopyranose side-chains. The splitting of three 13C signals in the regions of the 4- O-Man p units revealed that the α- d-Gal p units were irregularly (54%) distributed at the main chain. By rheology, GPC-coupled to multi-detectors, static and dynamic light scattering it was observed that the GM in nitrite solution presented an intrinsic viscosity of 860 mL/g, a polydispersity of 1.55, and a structural parameter ρ ( R g/ R h) of 1.77, showing a random coil and flexible conformation, which was confirmed by the Mark–Houwink constant α (0.63). The results suggest that GMPF can be a new source of GM.