Rotating type viscometer having a motor (2) for driving a rotating assembly (3) extending through an aperture (4) into a liquid to be tested (1) and measuring the shear forces imparted by said liquid (1) to the rotating assembly (3) . In order to prevent said liquid (1) from leaking through the aperture (4) due to a liquid pressure (p1) at the level of the aperture (4), the viscometer comprises means for creating a counter-pressure (p2) at the level of a gap between the rotating assembly (3) and the aperture (4). Thanks to this counter-pressure (p2), no solid sealing means are required, thereby improving the durability and the robustness of the viscometer, as well as simplifying it by eliminating countermeasures otherwise needed for taking into account parasitic friction forces exerted on the rotating assembly (3) due to such solid sealing means .