ObjectiveTo investigate cross-sectional associations between loneliness and health, health behaviours, and perceptions in Finnish individuals with overweight or obesity (BMI ≥25 kg/m2). MethodsWe used baseline data from patients participating, in 2016–2022, in a real-life digital 12-month weight management program known as Healthy Weight Coaching. Patients completed several questionnaires such as those related to loneliness, healthcare resource utilization, physical activity, and life satisfaction. BMI was computed based on self-reported weight and height. In addition to investigating individual health variables, we studied the association between loneliness and factor-analysis-derived health and wellbeing clusters. ResultsData were available from 2000 individuals (16.7% men, median age 48 years, median BMI 39.2 kg/m2). Altogether, 11.6%, 42.4%, and 46.0% reported feeling lonely, somewhat lonely, and not lonely, respectively. Feeling lonely was associated with higher BMI, greater healthcare resource utilization, lower life satisfaction, burdensomeness of life, more negative perceptions related to obesity and to the upcoming coaching, lower daytime energy, and reduced 20-min brisk walk results, a measure of functional capacity. Of the five factor-analysis-derived clusters, loneliness was adversely associated with “Life satisfaction” [lonely, 0.337 (0.270–0.421), p < 0.001; somewhat lonely, 0.545 (0.475–0.625), p < 0.001]. Moreover, loneliness associated with “Negative perceptions of obesity/daytime fatigue” [lonely, 4.627 (3.391–6.314), p < 0.001; somewhat lonely 2.021 (1.694–2.412), p < 0.001], and “Obesity/low physical activity” [lonely, 1.474 (1.105–1.966), p = 0.008; somewhat lonely, 1.220 (1.019–1.460), p = 0.030]. ConclusionsLoneliness had several untoward associations with health, health behaviours, and perceptions. Further research should explore the intricate relationship between obesity, loneliness, and physical and psychosocial health. Trial registrationThe trial is registered at clinicaltrials.cov (Clinical Trials Identifier NCT04019249).