We present the wave equation‐based adjoint tomography of northeastern Japan using common‐source double‐difference travel‐time data. More than 30,000 high‐quality first P‐wave arrivals from 117 local earthquakes recorded by 713 seismic stations are included in the inversion, generating more than 350,000 double‐difference travel times of first P‐wave arrivals. In comparison with commonly used travel‐time data, the common‐source double‐difference travel times are insensitive to source parameters and source‐side structures but place enhanced constraints on receiver‐side structures. As a result, these robust double‐difference travel times generate a reliable model down to a depth of 120 km. Our inversion results clearly show crustal low‐velocity anomalies beneath the volcanic arc. The subducting Pacific slab is illuminated as a landward‐dipping high‐velocity anomaly, and the arc magmatism in the mantle wedge is shown as low‐velocity anomalies. Thanks to the newly operated Seafloor Observation Network (S‐net), prominent high‐velocity bodies are detected in the fore‐arc mantle wedge at depths of 30–60 km. These abnormal high‐ bodies indicate a low degree of serpentinization in the fore‐arc mantle wedge at low temperatures, correlating well with the previous finding that the uppermost mantle in the Kuril and Tohoku fore‐arc is cold and dry. Our inversion results reveal reliable tectonic features in the subduction zone beneath northeastern Japan, suggesting that wave equation‐based common‐source double‐difference travel‐time adjoint tomography is an effective and robust method to illuminate detailed structures of the crust and uppermost mantle.