Teleseismic P waves are dominated by relatively low frequency (<1 Hz) signals due to the Earth's attenuation; because of this, receiver site effects are usually assumed to be independent of frequency. We investigate the site amplitude effects due to a surface sediment layer and show that teleseismic frequency-dependent site effects can be important. One-dimensional modeling shows that the effects of amplification due to differences in impedance, combined with amplitude losses to reverberations in the sediment layer, can result in significantly frequency-dependent site effects. The extent of the frequency dependence depends upon the two-way travel time of P waves in the sediments and the dominant period of the P waves. Using model CRUST2.0, we compute a 2¿ ¿ 2¿ map of the reference period of P waves above which the dependency can be neglected. We argue that the common practice of applying station corrections to magnitude determination will underestimate the magnitude of large earthquakes. To show that these effects are observable in practice, we analyze the amplitude ratios of seven teleseismic events at the station pair KURK and MAKZ and six events at the station pair KMI and CHTO. The amplitude ratios are relatively insensitive to other contributing effects and are in good agreement with the predicted frequency-dependent effects of the surface sediment layer.