In literature, most studies have only focused on sorption characteristics of quinoa grains. This study is one of the limited studies that deal with the sorption properties of quinoa with a thermodynamic approach. In the designed study, the equilibrium moisture content of whole quinoa grains determined by the static gravimetric method and ten different sorption models were applied for the description of the relationship between detected water activity and equilibrium moisture content. The thermodynamical properties of sorption have been determined by calculation of isosteric heat, entropy, Gibb’s free energy and spreading pressure. The best fitted sorption models are Brunauer–Emmett–Teller, Henderson, Peleg and Smith according to the statistical significance parameters. The monolayer moisture content of quinoa grains varies between 3.72–5.25%. The adsorption isotherm of grains are classified in Type-II by giving a sigmoidal shape. According to the thermodynamical analysis, the increase in moisture content leads to a decrease in isosteric heat of sorption from 50.4 kJ/mol to 43.7 kJ/mol. Additionally, the reduction in sorption heat is more gradual after 8% moisture content. Like isosteric heat of sorption, sorption entropy decreases from 15.9 J/molK to 1.5 J/molK with increasing water activity. Spreading pressure which means the surface excess-free energy, has ranged between 0.01 and 0.15 J/m2 and increase with water activity. Increasing moisture contents and temperatures have also decreased Gibb’s free energy level.