Humans and dogs have been living close for over 15,000 years, resulting in a unique relationship. The dog-owner relationship can affect stress, and could be affected by contact-seeking behaviour, personalities, and behavioural synchronization. Due to Covid-19, an increase in registered dogs could be observed. This could lead to more dogs being rehomed once the restrictions are eased and people go back to their workplaces, hence the importance of more knowledge concerning rehomed dogs and their behaviour, stress, and the relationship to its new owner. Thirty rehomed dogs (26 owners) and 31 control dogs (28 owners) participated. Dogs and their owners were tested in an unsolvable problem task to observe contact-seeking behaviour, and a behavioural synchronization test was conducted. Afterwards, dogs were equipped with activity collars to measure physical activity and dog-hair was collected to assess long-term stress. In addition, owners completed three questionnaires to assess the dog-owner relationship, and the personalities of the dogs and their owners. The results revealed no significant differences in behavioural synchronization, contact-seeking behaviour, stress, physical activity, or the dog-owner relationship between rehomed and control dogs. However, owners owning a rehomed dog potentially scored higher on the personality trait “agreeableness”, and rehomed dogs were potentially scored as more fearful of handling and less playful than control dogs. In conclusion, there is potentially a difference in personality traits in both dogs and owners between the groups, however, rehomed dogs are similar to control dogs in terms of behavioural synchronization, contact-seeking behaviour, long-term stress, and the dog-owner relationship.