Mortars are traditionally made from a mixture of sand, a binder and water. However, modern factory-made mortars are currently, very complex materials. Indeed, to exhibit various properties from the fresh paste to the hardened material, mortar formulations are composed of many mineral and organic admixtures. Among organic admixtures, polysaccharides, such as cellulose ethers (CE), are widely used into mortar formulation to improve water retention capacity of the freshly-mixed materials. The water retention capacity is an essential property of mortars leading to enhance the cement hydration and its adhesion to a substrate. Moreover, many polysaccharide admixtures, acting as viscosity-enhancing agents (VMA), prevent segregation and improve the homogeneity and workability of cement-based system. Indeed, the viscosity of the system strongly increases using polysaccharides and the rheological behavior of the cementitious paste may be completely modified and adapted to the application. Nevertheless, polysaccharides, as sugars, act on cement hydration. The main drawback is the retarding effect in hydration mechanism and setting-time of the cement. The present study examines the influence of chemical composition and structure of guar gum derivatives on fresh state properties of Portland cement-based mortars, such as water retention capacity (WR) and rheological behavior of fresh state Portland-based mortars. This work focuses on the guar gum derivatives since their manufacturing process is low pollutant and they provide very good properties to cement-based mortars. The investigation was also completed by adsorption isotherms. For this purpose, a native guar gum, some HydroxyProplyl Guars (HPG) and some hydrophobically modified HPG_s were selected. The effect of the molar substitution (MS_HP) and the degree of substitution (DS_AC) was investigated. The results highlight that chemical composition of HPGs has a remarkable effect on fresh state properties of mortars. The native guar gum does not impact both WR and rheological behavior. Increasing MS_HP leads to an improvement of the WR and the stability of mortars while the hydrophobic units further enhance WR and conduct to a decrease in the yield stress and an increase in the resistance to the flow of admixed mortars. Originality Hydroxypropyl guars (HPGs) are very interesting molecules because they are able to induce water retention properties comparable to those obtained with cellulose ethers, which are the widely used water-retaining agent. Moreover, by adapting the chemical composition and structure of the Hydroxypropyl guars it is possible to improve the water retention capacity of mortars, to modify the rheological behavior of mortars and to decrease the impact of the admixtures on the hydration delay of cement. It is important to underline that manufacturing of HPG causes less pollution than cellulose derivatives. Therefore the use of HPGs is in fact already well-established in construction chemistry. Nevertheless, studies about HPG as admixtures for cementitious materials are very scarce in the published literature.