This study focuses on the evaluation of teleseisms as estimators of long-period seismic site amplification in correspondence to large alluvial basins. Using the recordings of the 11 March 2011 M (sub w) 9.0 Tohoku-Oki, Japan, earthquake, the site response at low frequency of the Po Plain, Italy, was investigated. The interdistances among the analyzed sites, compared to the travel paths of the Tohoku-Oki waveforms with respect to the target area, allowed us to investigate the reliability of the standard spectral ratio (SSR) technique applied to teleseismic recordings. The results are presented as directional SSRs calculated considering 2 min of P, S, and Rayleigh (LR) waves, filtered from 0.025 to 1.0 Hz. Considering the horizontal components, at frequency from 0.04 to 0.3 Hz, the Po Plain stations show amplification factors up to 20 and 16, detected for the S and LR phases, respectively. In some cases, the amplified ground motion appeared to be affected by polarization phenomena, in particular in the north-south direction. Moving from the center to the edge of the basin, the LR-wave amplification remarkably decreases, in particular for frequencies between 0.1 and 0.3 Hz; this was not observed for the S wave. Amplification factors 〉10 were also detected for the vertical component, with increasing resonance periods moving from P to LR waves. Comparing SSRs with the single-station horizontal-to-vertical spectral ratios, the latter case shows that the vertical amplifications lead to the partial masking of the actual response of a site, in particular for frequencies 〉0.15 Hz. Finally, the relative P-wave amplitudes were also investigated. The results of the spectral analysis and the correlation between the relative P-wave amplitudes and the geometry of the bedrock suggest that the proposed approach is appropriate for the investigation of long-period seismic site response in similar settings.