This research aimed to improve our understanding of how owners' beliefs and behaviour are associated with obesity in companion dogs. To do this, we employed new theoretical frameworks and integrated previously reported measures to curate a collection of brief, user-friendly self-report measures to assess owner factors. The reliability and validity of these was examined in two phases of empirical research, each with a cross-sectional questionnaire design that also examined the validity of assessing body condition score (BCS) from photographs submitted by owners. Phase 1 (n = 47 dog owners from France) found that the brief owner-report measures correlated with the long-form measures (all correlations except one exceeded r = 0.70). BCS as coded from photographs were highly correlated with a vet's assessment of the same dogs (r = 0.67). Phase 2 (n = 3339 dog owners from France, Germany, the UK, Italy, and Russia) investigated which measures are associated with obesity among companion dogs. Perceptions of the dog's vulnerability to the threat of obesity, perceived weight status, perceived costs associated with ownership, normative beliefs about feeding, social support from friends, and being in the precontemplation stage of change predicted BCS alongside demographic factors (e.g., dog's age, neutered status). Taken together, the findings provide a method for assessing a wide range of factors that may be associated with obesity among companion dogs and point to potential targets for interventions designed to reduce obesity.