These data are part of a study commissioned by Energinet Eltransmission A/S, to collect baseline acoustic detection data for harbour porpoise in a proposed offshore wind farm area approx. 30 km off the north coast of Zealand, Denmark. The full baseline report for the Hesselø Offshore Wind Farm (Hesselø OWF), named after an uninhabited island southwest of the area, can be found here: https://ens.dk/media/5429/download. Eight stations cover 13 months of monitoring (December 2020 to December 2021). One station was discontinued in June 2021 due to equipment/data loss presumably from trawling, while a new station was established in a safer location. Each deployment period is sequentially marked A, B, C, and so forth. Porpoises were found to be present year-round, with more detections at the northern stations. There were low levels of detections in summer at the southern stations, indicating this area is not an important breeding area, which is confirmed by the accompanying aerial surveys.