Seabirds have increasingly encountered ofshore wind farms (OWFs) in European waters in the past 10 years, resulting in potential conficts with ofshore foraging areas. During the breeding season, seabirds are restricted in their choice of foraging habitat and are under increased pressure to fnd enough prey to raise their ofspring. However, information on the individual reactions of seabirds towards OWFs during the breeding season is lacking. Three OWFs located 23–35 km north of the island of Helgoland have operated since October 2015. We studied their possible efects on locally breeding common guillemots (Uria aalge) using GPS tracking. GPS tags were deployed on 12 breeding guillemots from Helgoland for 8–26 days during 2016–2017. Most individuals avoided the OWFs, but one individual in each year briefy entered the OWFs on two or three occasions. Using a point process model, we revealed a 63% reduction in the resource selection of the OWF areas compared with the surroundings (lower confdence interval (CI) = 79% reduction, upper CI = 36% reduction). Furthermore, OWF avoidance was increased to 75% when the turbine blades were rotating (lower CI = 93% reduction, upper CI = 11% reduction). Guillemots mainly approached the OWFs from their eastern edge when resting or diving, and rarely approached the areas when commuting. These results provide a detailed description of guillemot reactions to OWFs during the breeding season, and the frst comprehensive analysis of OWF efects on this species based on telemetry data. The strong avoidance efect for guillemots during the breeding season indicates the need to consider the presence of OWFs when interpreting future trends in the abundance and breeding success of this species.