Much of the global canine population is now overweight, and this can adversely affect health, lifespan and quality of life. Undesirable behaviours are also common in pet dogs, and these can adversely affect welfare, as well as being stressful to owners. However, links between obesity and behavioural disorders have never previously been explored. An online survey was conducted between June and August in 2014, coinciding with the broadcast of a National UK television programme, exploring dog health, welfare and behaviour. Information gathered included signalment, overweight status and the prevalence of a range of undesirable behaviours. Fisher's exact test and OR were used to determine associations between overweight status and owner-reported behaviours. A total of 17028 responses were received. After data verification, the final dataset comprised 11154 dogs, 1801 (16.1%) of which were reported by owners to be overweight. Owners of overweight dogs were more likely to see them as 'a baby' (P<0.0001) and allow them to sleep on their bed (P<0.0001). Overweight dogs were also more likely to guard food (P<0.0001) and steal food (P<0.0001). Other undesirable behaviours more commonly reported in overweight dogs included barking, growling or snapping at strangers (P=0.0011) and other dogs (P=0.0015), being fearful of outdoors (P<0.0001), and not always coming back when called (P=00011). Finally, owners were more likely to report that unsociable behaviours adversely affected their dog's health (P<0.0001). Overweight status is associated with a number of undesirable behaviours in dogs. Further studies are now required to explore the reasons for these associations.