This paper investigates the effect of frequency-dependent soil parameters on the simulation of electromagnetic transients on overhead lines, based on the application of the FDTD method to solve the telegrapher’s equations. The ground return impedance is calculated assuming either constant or frequency-dependent soil parameters. Frequency-domain results show differences in the propagation velocity and attenuation constant when assuming either constant or frequency-dependent soil parameters. Time-domain simulations considering a 230-kV line indicate that the consideration of the frequency variation of soil electrical parameters can be relevant to the simulation of high-frequency transients on transmission lines for poorly conducting soils. It is also found that this variation changes not only the amplitude but also the waveform of the resulting transient voltages. The analysis presented indicates that EMT-like tools, which traditionally assume Carson’s formulation for the ground return impedance together with constant soil parameters, should be used with caution in lightning transient studies involving overhead lines above high-resistivity soils.