We evaluated the long‐period site response of peak ground velocities (PGVs) and peak ground displacements (PGDs) in the 5–30 s period range at 198 K‐NET, KiK‐net, and Japan Meteorological Agency strong‐motion stations in northeastern Japan. Long‐period site responses were estimated empirically based on the ratio of observed ground motions on free surfaces to the values predicted by ground‐motion prediction equations (GMPEs) on bedrock in which the shear‐wave velocity is ≥2000 m/s. Our results show large site amplifications generally dominate at basin stations, whereas site deamplifications generally dominate at mountain stations. The long‐period site response factors of PGVs and PGDs have ranges of 0.6–3.6 and 0.4–3.1, respectively, for which the factors of PGVs are larger than those of PGDs by an average factor of ∼1.3. Long‐period site responses were used to correct the observed strong ground motions of eight earthquakes. The long‐period site‐corrected data fit better with GMPEs that were inferred from smaller standard errors and show a better correlation with sediment thickness, and thus may contribute to reducing the variability of seismic‐hazard assessment.