The speed of torsional stress waves transmitted in solid waveguides submerged in a liquid depends, among other things, on the liquid's density and viscosity and the waveguides' cross-sectional geometry. By measuring the speed of torsional stress waves in two waveguides of different cross-sectional geometries, one can obtain both the liquid's density and viscosity. An online, real-time sensor for the simultaneous measurement of density and viscosity is described. The article details the sensor's principles of operation and reports experimental results conducted using viscosity standard calibration liquids with wen-known thermophysical properties. For fluids with density ρf > 1 X 103 kg/m3 , it is estimated that the instrument can measure density with a precision better than 0.5%. For fluids with the product shear viscosity (µ) and density, ρfµ> 100 kg2/(m4s), it can measure the shear viscosity with a precision better than 1%.