The Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S‐net) is a novel cabled ocean‐bottom station network covering a broad offshore region east of northeastern Japan. To best use the S‐net data, we estimated sensor orientations of all 150 S‐net stations, because without this information the orientations of measurements in geodetical coordinates cannot be specified. We determined three parameters of the sensor orientation at each station: the tilt angle of the long axis of the cable, the rotation angle around the long axis, and the azimuth of the long axis. We estimated the tilt and rotation angles by using the direct current components of accelerometers recording the gravitational acceleration. The tilt and rotation angles slightly varied within the range of 0.001°–0.1° for most stations during the period from 2016 to 2018 except for coseismic steps of rotation angles greater than 1° because of the 20 August 2016 6.0 off Sanriku and 20 November 2016 6.9 off Fukushima earthquakes. The long‐axis azimuths were estimated by the particle motions of long‐period Rayleigh waves. We used the accelerometer records in 0.01–0.03 Hz of 7–14 teleseismic earthquakes with 7.0–8.2. The azimuths were constrained with 95% confidence intervals of . After correcting original waveforms based on the estimated sensor orientation, we confirmed coherent waveforms within the whole S‐net stations and separation of Rayleigh and Love waves in radial and transverse components. The waveforms were also coherent with those of on‐land broadband stations. We provide the estimated sensor orientations and rotation matrix for conversion from the XYZ to east, north, and up components. The estimated orientation can be a fundamental resource for further seismic and geodetic explorations based on S‐net data.