Modern paper machines have reached their practical maximum width, because raising the width would require significant investments to eliminate vibrations of the cylinders at high speeds. To increase the amount of produced paper on a paper machine, web breaks, broke and downtime in general should be minimized and the production speed maximized. In many cases, the increase in paper machine production speed is limited by the mechanical properties of wet web and paper machines are thus forced to run below their design speed. Therefore obtaining knowledge about what kind of chemicals could be used to improve wet web mechanical properties would be valuable. Guar gum is a carbohydrate polymer containing galactose and mannose sugar units, generally containing one galactose unit for every two mannose units. Due to the unique rheology modifying properties of guar gum, it is widely used across a broad spectrum of industries, including in the paper industry. The conformation of guar gum is very similar to that of cellulose, resulting in strong adsorption onto cellulose fibers. Wet end addition of guar gum is known to enhance printability by giving a denser paper surface. Additionally, guar gum is known to improve filler retention and the strength properties of dry paper. However, there is much less information on the effect of guar gum on mechanical properties of wet paper web. In this paper the effect of guar gum was studied both on laboratory and pilot scale. In the laboratory examination, the effects of adding different commercial polymers by spraying to formed sheets of bleached hardwood kraft pulp on wet web tensile properties at a high strain rate were examined. Guar gum was found to be superior in improving wet web strength to other tested polymers, which were cationic starch, CMC, chitosan and PVA. The increase in wet web strength was found to be greatly affected by the molar mass of guar gum and by the dosing strategy (i.e. spray vs. pulp suspension addition). The results presented here give support to the idea that molecular level inter diffusion of fiber surfaces at contact areas already occurs in wet state and it has a great impact on the stress transfer between fibers at wet state. The inter diffusion and thus the strength of fiber contact areas, and eventually the fiber network can be tailored by adsorbing different polymers to fiber surface. The potential of using guar gum in practical paper and board making was verified in two pilot-scale trials. The results were promising, showing that applying guar gum by spraying caused no problems in drainage, dewatering during wet pressing, web handling or end product quality, and it improved the mechanical properties of dry and wet paper.