Background: Comorbidity in people with asthma can significantly increase asthma morbidity and lower adherence to asthma guidelines.Objective: The objective of this study was to comprehensively measure the prevalence of physical and mental health comorbidities in adults with asthma using a large nationally representative population.Methods: Cross-sectional analysis of routine primary care electronic medical records for 1,424,378 adults in the UK, examining the prevalence of 39 comorbidities in people with and without asthma, before and after adjustment for age, sex, social deprivation and smoking status using logistic regression.Results: Of 39 comorbidities measured, 36 (92%) were significantly more common in adults with asthma. 62.6% of adults with asthma had ≥1 comorbidity vs. 46.2% of those without, and 16.3% had ≥4 comorbidities vs. 8.7% of those without. Comorbidities with the largest absolute increase in prevalence in adults with asthma were: COPD (13.4% vs 3.1%), depression (17.3% vs 9.1%), painful conditions (15.4% vs 8.4%), and dyspepsia (10.9% vs 5.2%). Comorbidities with the largest relative difference in adults with asthma compared to without were: COPD (adjusted odds ratio [aOR] 5.65, 95%CI 5.52-5.79), bronchiectasis (aOR 4.65, 95%CI 4.26-5.08), eczema/psoriasis (aOR 3.30, 95%CI 3.14-3.48), dyspepsia (aOR 2.20, 95%CI 2.15-2.25) and chronic sinusitis (aOR 2.12, 95%CI 1.99-2.26). Depression and anxiety were more common in adults with asthma (aOR 1.60, 95%CI 1.57-1.63, and aOR 1.53, 95%CI 1.48-1.57 respectively).Conclusions and Clinical Relevance: Physical and mental health comorbidity is the norm in adults with asthma. Appropriate recognition and management should form part of routine asthma care.