An undergraduate experiment is described for determining thermodynamic temperature directly from the definition of entropy. A simple system is arranged so that the entropy change due to the addition of heat may be calculated from the resulting change in the pressure exerted by the system. For this calculation the isentropic thermal expansion of the system must be determined experimentally. It is shown that in principle the temperature as determined does not require calibration of the thermometer used except at two fixed points. Accuracy of the order of 1 percent can be attained.