Introduction As a common chronic disease seen across all ages, asthma has the potential to incur high societal and individual costs from both direct healthcare costs and loss of productivity. Most previous studies use smaller, selected populations to assess the cost of asthma, possibly reducing generalisability. We, therefore, aimed to assess the total, nationwide economic burden of asthma by severity from both an individual and a societal perspective. Methods The annual cost of asthma was assessed in a Danish nationwide cohort of patients aged 18-45 during 2014-2016 as excess healthcare costs, loss of income and welfare expenditure compared with controls (matched 1:4) using national registries. Asthma severity was defined as mild-to-moderate (steps 1-3 or step 4 without exacerbations) or severe (step 4 with exacerbations or step 5). Results Across 63 130 patients (mean age 33, 55% female), the annual excess cost of asthma compared with controls was predicted to €4095 (95% CI €3856 to €4334) per patient. Beyond direct costs related to treatment and hospitalisations (€1555 (95% CI €1517 to €1593)), excess indirect costs related to loss of income (€1060 (95% CI €946 to €1171)) and welfare expenditure (eg, sick pay and disability pensions) (€1480 (95% CI €1392 to €1570)) were seen. Crude pooling of excess costs resulted in an annual societal cost of €263 million for all included patients. Severe asthma (4.5%) incurred 4.4 times higher net costs (€15 749 (95% CI 13 928 to €17 638)) compared with mild-to-moderate disease (€3586 (95% CI €3349 to €3824)). Furthermore, patients with severe asthma experienced an annual loss of income of €3695 (95% CI €4106 to €3225) compared with controls. Conclusion In young adults with asthma, a significant societal and individual financial burden of disease was seen across severities. Expenditure was mainly driven by loss of income and welfare utilisation, rather than direct healthcare costs.