trans-1,3,3,3-Tetrafluoropropene (HFO-1234ze(E)) is considered as an alternative refrigerant in automobile air conditioning applications because of its low global warming potential. For the purpose of evaluation of thermophysical properties of HFO-1234ze(E), the speed of sound was measured in the dilute gas region in order to derive heat capacities in the ideal gas state. The speed of sound was obtained from measurements of acoustic resonance frequencies of radial modes in a spherical resonator filled with sample gas. Taking some perturbation effects into account, the speed of sound was determined with a relative uncertainty of 0.01 %. The speed of sound data were fitted to the acoustic virial equation. By extrapolating the speed of sound data on each isotherm to zero pressure, the ideal gas heat capacities at constant pressure were determined with a relative uncertainty of 0.1 %. The isobaric ideal gas heat capacities were represented by a third-order polynomial function in temperature.