The viscosity of nanofluids, and consequently their energy dissipation, is a challenging factor in real applications because it affects the pressure drop and pumping power of equipment in industries. The aim of this experimental and simulation study is to measure and calculate the viscosity of a carbon nanotube (CNT)-water nanofluid by employing a rotational viscometer and by molecular dynamics (MD) simulation. The effects of temperature, solid concentration, and CNT diameter on the dynamic viscosity were examined within the temperature and volume concentration ranges of 25-65 degrees C and 0.125%-1%, respectively. Interestingly, the maximum observed increase in the relative viscosity of CNT-water nanofluid occurred at 65 degrees C, whereas the absolute viscosity of the nanofluid was minimum at this temperature. It was further found that the dynamic viscosity increases with increasing volume fraction of nanoparticles and with decreasing nanofluid temperature, whereas changing the diameter of the CNT does not have a significant effect on nanofluid viscosity. Furthermore, a correlation function was proposed in terms of solid concentration and nanofluid temperatures based on the MD simulation results, and its accuracy was investigated by analyzing the margin of deviation. The findings of this study are useful for industrial applications of CNT/water nanofluids.