Grain boundary sliding (GBS) was observed at room-temperature, at a very early stage of tensile tests on Grade 2 and Grade 4 titanium specimens covered with fiducial grids. AFM images were also repeatedly captured to monitor the evolution with strain of the out-of-plane component of GBS. Sharp gradients in sliding were observed along some grain boundaries, which proved that GBS should not be considered as a rigid-body motion. Grade 4 Ti was less prone to GBS than Grade 2, which was attributed to a pinning effect of βphase particles present along the grain boundaries. Contrary to what was reported for Zinc and Magnesium alloys, GBS was not found to contribute significantly to the global plastic strain in Titanium at room temperature. Crystal viscoplasticity finite element simulations based on real microstructure were run to compute the shear stress along the grain boundaries for various strain rates. The shear stress along the boundaries that slid was found to be substantially higher than the mean shear stress over all boundaries. However, a high shear stress was not sufficient to trigger GBS, also influenced by other factors. A change in strain rate not only modified the mean shear stress on all grain boundaries, but also the relative values of the shear stress on the grain boundaries, which was attributed to the strain-rate dependence of slip activities in each grain.