The effect of liquid viscosity on the penetration and trajectory of a jet in a low subsonic cross-airflow was investigated experimentally. An open-loop wind tunnel was used to generate an airstream in a square cross-sectional test section. Liquid was injected downward through a nozzle that was flush with the top inner surface of the test section. A wide range of experimental conditions was achieved by varying the nozzle diameter, momentum flux ratio, and liquid viscosity. The study revealed that viscosity has distinct effects on the initial part of the liquid column and in the jet's far-field stream. It was shown that far from the nozzle exit, the jet's penetration increased initially as the liquid viscosity increased, but a further increase in viscosity reduced the penetration. On the other hand, close to the nozzle exit, although the effect of liquid viscosity was not obvious, it was genera ally observed that with the exception of the highest viscosity employed here, the jet's penetration decreased as the viscosity increased. An empirical jet trajectory correlation was proposed to account for the combined effects of viscosity, momentum flux ratio, and nozzle diameter.