We found offshore shallow low‐frequency tremor in the northern Japan Trench subduction zone, using records from the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench. The tremor aligned with a narrow zone, which follows the 10‐ to 25‐km depth contours of the plate interface and occurred in two distinct areas, off Tokachi and Sanriku, separated by a gap. In the 1.75 years examined, two episodes lasting for a few weeks were identified off Tokachi. During these episodes, tremor occurred concurrently with swarms of very low frequency earthquakes and migrated laterally about 120 km along the trench strike. In contrast, tremor off Sanriku lasted shorter (half a day to 5 days) and repeated at shorter intervals (1–4 months). The variation in tremor behavior such as duration, recurrence, migration, and the association with very low frequency earthquakes suggests along‐strike heterogeneities in frictional properties at the shallow plate interface. Plain Language Summary Low‐frequency tremor has been widely recognized at many subduction zones yet has not been directly measured in the Japan Trench subduction zone in northeastern Japan. Using a newly installed seafloor seismic network, we report for the first time observations of tremor occurring at the shallow subduction plate interface in the northern Japan Trench subduction zone. The observed tremor, localized in the regions off Tokachi and Sanriku, were synchronized in space and time with very low frequency earthquakes and moved across the region during active periods. Tremor off the Tokachi and Sanriku regions had different characteristics, including duration and recurrence time. This difference is likely caused by different frictional properties at the plate interface near the trench. Key Points We identified offshore shallow low‐frequency tremor parallel to the trench strike in the northern Japan Trench subduction zone using S‐net Tremor occurred in the two clusters separated by a gap and coincided with very low frequency earthquakes Spatial differences in tremor behavior imply lateral heterogeneities in frictional properties at the shallow plate interface