In earthquake engineering, “engineering bedrock” is regarded as a stiff material (i.e., rock or rock‐like geological formation) that is characterized by a shear‐wave velocity greater than a target value (e.g., ; current Italian and European seismic codes). In the case of deep basins, the identification of engineering bedrock is problematic, because it can lie well below the penetration depth of most common prospecting methods (i.e., a few tens of meters). Moreover, the depth of engineering bedrock might not represent an effective proxy of the sedimentary thickness responsible for site amplification. The Po Plain sedimentary basin (northern Italy) is one of the deepest and widest worldwide, and it presents such problems. The aim of this work is to estimate the sedimentary thickness responsible for ground‐motion amplification at medium and long periods in the Po Plain. Passive seismic prospecting methods based on ambient‐vibration measurements using single‐station and array configurations were considered to map “seismic bedrock” depth. This corresponds to a marked seismic impedance contrast where the shear‐wave velocity approached, or exceeded, . In the latter case, seismic and engineering bedrocks coincided. Our mapping will be useful for future site response assessments, numerical modeling of seismic‐wave propagation, dynamic ground response analyses, and site‐specific seismic hazard evaluation at the basin scale.