Site amplification is an important factor to determine seismic‐wave amplitude in addition to source and propagation factors. Many previous studies have estimated site amplification factors in the frequency domain. In recent decades, real‐time prediction of strong ground motion has been widely investigated and applied for earthquake early warning. Frequency‐dependent site amplification factors, however, have not yet fully been taken into account in studies of the real‐time prediction. A method for real‐time correction of frequency‐dependent site factors is proposed in this paper, in which the frequency dependence is reproduced by a causal recursive filter in the time domain.