HFE 7100 (C4F9OCH3) is a promising working fluid for organic Rankine cycles (ORC). However, there are no available published experimental gaseous pvT data. Thus, pvT data of HFE 7100 in the gaseous phase were measured with a Burnett apparatus. High purity helium was used to calibrate the cell constant of the Burnett apparatus. Then, the gaseous pvT data of HFE 7100 were measured from (363 to 431) K for pressures from (148 to 878) kPa. The temperature measurement standard uncertainty was estimated to be 2.3 mK, the pressure standard uncertainty was estimated to be 240 Pa, and the density relative standard uncertainty was estimated to be 0.0006. A truncated virial equation of state (EOS) for HFE 7100 was used to correlate the experimental data with a relative root-mean-square deviation of 0.00042.