The Thessaloniki Metropolitan Railway is currently at its primary construction stage. The railway is composed of two separate ∼6-m-diameter parallel tunnels, each of which is ∼8 km long and has 13 stations. The span between the tunnel axes is approximately 12 m, and the depth of the red line (track level) varies from 15 m to 30 m. The geology of the urban area of Thessaloniki is characterized by the presence of Neogene and Quaternary deposits. The base formation for the study area is a very stiff to hard red clay, dating to Upper Miocene–Pliocene time. On top of this formation, Quaternary sediments have been deposited, most of which consist of sand and/or gravel in a clay-silt–dominated matrix, covered in places by anthropogenic fill. Ground investigation campaigns have incorporated a significant number of sampling boreholes and in situ and laboratory testing. In this study, data from borehole loggings, particle-size analyses, natural moisture content, Atterberg limits, permeability tests, pressure-meter tests, cone penetration tests (CPT), and standard penetration tests (SPT) were analyzed in order to obtain a better geological understanding, a geotechnical zonation, and a classification of the ground with respect to mechanized tunneling. The characteristics and parameters of the soils and the hydrogeological regime indicate that an EPBM (Earth Pressure Balance Machine) rather than a slurry TBM (Tunnel Boring Machine) should be used for the tunnel construction.