Having a reliable site classification scheme is vital for the development of robust strong-motion attenuation models. We discuss a promising empirical site-classification scheme based on strong-motion data from Japan. We assigned site classes, according to the site classification defined in engineering design codes in Japan, for those K-net sites where boreholes reached either to rock or to stiff soils with shear-wave velocity of 600 m/sec or larger, using four site classes defined by dominant site period. The average response spectral ratios of the horizontal and vertical components (H/V) of earthquake records for all site classes were found not to be strongly affected by Japan Meteorological Agency (JMA) magnitude, hypocentral distance, and focal depth for all site classes. We used H/V ratios for records from the classified K-net sites to establish a site classification index using the mean spectral ratios over a wide range of spectral period. Using the index, we were able to classify both K-net stations with soil layers thicker than 20 m and other strong-motion stations in Japan. The peak period of the H/V spectral ratio can also be used to identify soft soil sites. The site amplification factors calculated from the site class terms based on the new site classification are consistent with the period bands defined for these site classes.