The effect of dissolved water on hydrocarbon viscosity at high temperatures and high pressures (HPHT) is of interest because water is part of the reservoir fluid in some forms, and dissolved water concentration at these conditions can be as high as 0.1 mole fraction and cannot be neglected. In this work, a series of comprehensive and systematic viscosity experiments to extend the experimental viscosity data of hydrocarbon fluids with the presence of dissolved water at HPHT conditions were performed. The main objective of this investigation is to test if the dissolved water in hydrocarbon can change the viscosity of reservoir fluids at high-temperature and high-pressure conditions. Binary mixtures of methane with n-heptane, n-decane, and toluene and also three multicomponent systems: gas condensate, natural gas, and a synthetic volatile oil which were mixed with various amounts of water (0.02 mole fraction, 0.05 mole fraction, and saturated with water) were tested. The viscosity measurements were conducted in a HPHT PVT cell equipped with capillary tube at pressures and temperatures up to 138 MPa and 473.15 K, respectively.