Offshore pile driving, e.g. during wind farm construction, produces substantial noise emissions into the water column, which may harm marine mammals. Therefore, it is common practice to attempt to deter the mammals out of potential danger zones beforehand. Seal scarers are commonly used as a deterrent for harbour porpoises in spite of a lack of clear evidence in support of their effectiveness. We investigated the responses of harbour porpoises to a Lofitech seal scarer by conducting visual observations in conjunction with sound measurements. Porpoise sighting rates within 1 km of the seal scarer significantly decreased to only 1% during seal scarer activity. During 22 trials, when the seal scarer was deployed between 300 m and 3.3 km distance, all observed porpoises always avoided the seal scarer within 1.9 km (translating to sound levels of ≥122 dB re 1 µParms), avoided the seal scarer half the time within 2.1 to 2.4 km (119 to 121 dB re 1 µParms) and never avoided the seal scarer at distances beyond 2.6 km (≤118 dB re 1 µParms). The closest observed approach distance of a porpoise to the activated seal scarer was 798 m (132 dB re 1 µParms). Thus, the deployment of a Lofitech seal scarer during offshore pile driving activities can greatly reduce the risk of acoustic traumata to harbour porpoises. However, danger zones and thus the necessary deterrence zones have to be calculated specifically for each project based on measurements of sound transmission in the area.