The Groningen gas field is the largest gas field in Europe. Gas production in this field has led to seismicity and surface subsidence, both believed to be caused by compaction of the underlying reservoir sandstone. In 2015, the field operator (Nederlandse Aardolie Maatschappij - NAM) installed a fibre optics cable in the Zeerijp-3a well, at a true vertical depth of about 2900 to 3200 m, i.e. in and around the gas reservoir. The Zeerijp-3a well is situated in the center of the field, where seismicity (<3.4 M) and subsidence (up to 35 cm) are both greatest. The fibre optics cable allows real-time, continuous, in-situ monitoring of compaction of the reservoir and the over- and underlying formations, through the Distributed Strain Sensing (DSS) technique. DSS data (strain-time-depth) obtained from October 2015 to December 2021 are provided by NAM, open access. The data were processed by NAM, as detailed in the report accompanying the data. Raw data are available on request. The data presented in this data ...