A method and a device for the online determination of the viscosity of a polymer in pasty to liquid form undergoing processing, such as extrusion is disclosed. It is provided that for the online determination of the viscosity of the polymer, at least a portion of the polymer undergoing processing is diverted and conveyed to the measurement volume of a measuring module, a predetermined volume of the respective batch is expelled from the measurement volume through a measurement nozzle by subjecting the batch to a predetermined pressure, the time required for expelling the predetermined volume of the batch is measured, the measured values are used for calculating the viscosity of the polymer, and prior to the filling of the measurement volume with the polymer to be measured, the measurement volume is flushed at least once with a quantity of the polymer being processed.